来啊踩fastjson打印入参导致业务跑偏的坑

　　线上代码对日志的记录，重要性自不必说。但是怎样记录日志也是有讲究的！
　　日志可以直接在每个方法中进行日志记录，优点是想怎么记就怎么记，缺点是记日志的代码可能会超过你的业务代码，可读性急剧下降，这也是日志框架蓬勃发展的源头。
　　日志也可以通过非业务代码侵入的形式进行记录，具体来说就是合作切面（aop）进行日志记录，好处自然是让业务更纯洁，缺点是运行性能会受到影响。
　　当然了，前面这些都不是本文的主题。本文的主题是，当你用切面进行日志记录时，然后fastjson是如何把你的业务代码给干掉的。
　　JsonObject.toJsonString(pj.getArgs()); 这是一个对入参的一个简单打印，至于使用fastjson的原因，自然是因为其输出字符的易读性。args是一个数组。那么这里会有什么问题？普通的参数自然是没有问题了。问题在于，fastjson转换对象为字符串时，报错了。为什么会这样？

　　注： 本文bug所现问题入参类型示例为：

public Object doSth(@RequestBody InBean in, HttpServletRequest request) {
     // do sth...
}

　　切面报错为：

public Object deal(ProceedingJoinPoint pjp) throws Throwable {
      String argsStr = JSONObject.toJSONString(pjp.getArgs());
      logger.info("入参:{}", argsStr);
}

解析下fastjson源码过程：

初步进入源码：

// 直接调用json化方法
    public static String toJSONString(Object object) {
        return toJSONString(object, emptyFilters);
    }

// 转到复杂参数的调用
    public static String toJSONString(Object object, SerializeFilter[] filters, SerializerFeature... features) {
        return toJSONString(object, SerializeConfig.globalInstance, filters, null, DEFAULT_GENERATE_FEATURE, features);
    }

// 最终转换方法，1. 获取writer; 2. 处理filter; 3. 将数据定稿writer缓冲; 4. 返回缓冲数据给调用方
    public static String toJSONString(Object object, //
                                      SerializeConfig config, //
                                      SerializeFilter[] filters, //
                                      String dateFormat, //
                                      int defaultFeatures, //
                                      SerializerFeature... features) {
        SerializeWriter out = new SerializeWriter(null, defaultFeatures, features);

        try {
            JSONSerializer serializer = new JSONSerializer(out, config);

            if (dateFormat != null && dateFormat.length() != 0) {
                serializer.setDateFormat(dateFormat);
                serializer.config(SerializerFeature.WriteDateUseDateFormat, true);
            }

            if (filters != null) {
                for (SerializeFilter filter : filters) {
                    serializer.addFilter(filter);
                }
            }

            serializer.write(object);        // 最核心方法

            return out.toString();
        } finally {
            out.close();
        }
    }

深入内部调用，追根究底：

// 写方法的进一步操作
    public final void write(Object object) {
        if (object == null) {
            out.writeNull();
            return;
        }

        Class<?> clazz = object.getClass();
        ObjectSerializer writer = getObjectWriter(clazz);    // 关键,返回各种对应的序列化器, 如 ObjectArrayCodec

        try {
            writer.write(this, object, null, null, 0);        // 调用对应的序列化器进行处理
        } catch (IOException e) {
            throw new JSONException(e.getMessage(), e);
        }
    }

// 追踪到在运行write方法时出的问题 ObjectArrayCodec
    public final void write(JSONSerializer serializer, Object object, Object fieldName, Type fieldType, int features)
                                                                                                       throws IOException {
        SerializeWriter out = serializer.out;

        Object[] array = (Object[]) object;

        if (object == null) {
            out.writeNull(SerializerFeature.WriteNullListAsEmpty);
            return;
        }

        int size = array.length;

        int end = size - 1;

        if (end == -1) {
            out.append("[]");
            return;
        }

        SerialContext context = serializer.context;
        serializer.setContext(context, object, fieldName, 0);

        try {
            Class<?> preClazz = null;
            ObjectSerializer preWriter = null;
            out.append('[');

            if (out.isEnabled(SerializerFeature.PrettyFormat)) {
                serializer.incrementIndent();
                serializer.println();
                for (int i = 0; i < size; ++i) {
                    if (i != 0) {
                        out.write(',');
                        serializer.println();
                    }
                    serializer.write(array[i]);
                }
                serializer.decrementIdent();
                serializer.println();
                out.write(']');
                return;
            }

            for (int i = 0; i < end; ++i) {
                Object item = array[i];

                if (item == null) {
                    out.append("null,");
                } else {
                    if (serializer.containsReference(item)) {
                        serializer.writeReference(item);
                    } else {
                        Class<?> clazz = item.getClass();

                        if (clazz == preClazz) {
                            preWriter.write(serializer, item, null, null, 0);
                        } else {
                            preClazz = clazz;
                            preWriter = serializer.getObjectWriter(clazz);

                            preWriter.write(serializer, item, null, null, 0);
                        }
                    }
                    out.append(',');
                }
            }

            // 最后一个数组需要单独处理
            Object item = array[end];

            if (item == null) {
                out.append("null]");
            } else {
                if (serializer.containsReference(item)) {
                    serializer.writeReference(item);
                } else {
                    serializer.writeWithFieldName(item, end);
                }
                out.append(']');
            }
        } finally {
            serializer.context = context;
        }
    }

找到最后个出现问题的地方，发现再往下无法再跟踪：

// 外部简单调用
public final void writeWithFieldName(Object object, Object fieldName) {
    writeWithFieldName(object, fieldName, null, 0);
}

// 最终报错是在这里
    public final void writeWithFieldName(Object object, Object fieldName, Type fieldType, int fieldFeatures) {
        try {
            if (object == null) {
                out.writeNull();
                return;
            }

            Class<?> clazz = object.getClass();

            ObjectSerializer writer = getObjectWriter(clazz);        // ASMSerializer_2_RequestFacade 

            writer.write(this, object, fieldName, fieldType, fieldFeatures);    // 此处使用asm语言写的代码，无法debug, 但是会抛出 IllegalStateException
        } catch (IOException e) {
            throw new JSONException(e.getMessage(), e);
        }
    }

检查最后一次出现的问题，搜索出现的标记，找到动态生成asm代码的文件：

// 动态生成代码 ASMSerializer_2_RequestFacade
    public JavaBeanSerializer createJavaBeanSerializer(SerializeBeanInfo beanInfo) throws Exception {
        Class<?> clazz = beanInfo.beanType;
        if (clazz.isPrimitive()) {
            throw new JSONException("unsupportd class " + clazz.getName());
        }

        JSONType jsonType = clazz.getAnnotation(JSONType.class);

        FieldInfo[] unsortedGetters = beanInfo.fields;;

        for (FieldInfo fieldInfo : unsortedGetters) {
            if (fieldInfo.field == null //
                && fieldInfo.method != null //
                && fieldInfo.method.getDeclaringClass().isInterface()) {
                return new JavaBeanSerializer(clazz);
            }
        }

        FieldInfo[] getters = beanInfo.sortedFields;

        boolean nativeSorted = beanInfo.sortedFields == beanInfo.fields;

        if (getters.length > 256) {
            return new JavaBeanSerializer(clazz);
        }

        for (FieldInfo getter : getters) {
            if (!ASMUtils.checkName(getter.getMember().getName())) {
                return new JavaBeanSerializer(clazz);
            }
        }

        String className = "ASMSerializer_" + seed.incrementAndGet() + "_" + clazz.getSimpleName();
        String packageName = ASMSerializerFactory.class.getPackage().getName();
        String classNameType = packageName.replace('.', '/') + "/" + className;
        String classNameFull = packageName + "." + className;

        ClassWriter cw = new ClassWriter();
        cw.visit(V1_5 //
                 , ACC_PUBLIC + ACC_SUPER //
                 , classNameType //
                 , JavaBeanSerializer //
                 , new String[] { ObjectSerializer } //
        );

        for (FieldInfo fieldInfo : getters) {
            if (fieldInfo.fieldClass.isPrimitive() //
                //|| fieldInfo.fieldClass.isEnum() //
                || fieldInfo.fieldClass == String.class) {
                continue;
            }

            new FieldWriter(cw, ACC_PUBLIC, fieldInfo.name + "_asm_fieldType", "Ljava/lang/reflect/Type;") //
                                                                                                           .visitEnd();

            if (List.class.isAssignableFrom(fieldInfo.fieldClass)) {
                new FieldWriter(cw, ACC_PUBLIC, fieldInfo.name + "_asm_list_item_ser_",
                                ObjectSerializer_desc) //
                                                       .visitEnd();
            }

            new FieldWriter(cw, ACC_PUBLIC, fieldInfo.name + "_asm_ser_", ObjectSerializer_desc) //
                                                                                                        .visitEnd();
        }

        MethodVisitor mw = new MethodWriter(cw, ACC_PUBLIC, "<init>", "(" + desc(SerializeBeanInfo.class) + ")V", null, null);
        mw.visitVarInsn(ALOAD, 0);
        mw.visitVarInsn(ALOAD, 1);
        mw.visitMethodInsn(INVOKESPECIAL, JavaBeanSerializer, "<init>", "(" + desc(SerializeBeanInfo.class) + ")V");

        // init _asm_fieldType
        for (int i = 0; i < getters.length; ++i) {
            FieldInfo fieldInfo = getters[i];
            if (fieldInfo.fieldClass.isPrimitive() //
//                || fieldInfo.fieldClass.isEnum() //
                || fieldInfo.fieldClass == String.class) {
                continue;
            }

            mw.visitVarInsn(ALOAD, 0);

            if (fieldInfo.method != null) {
                mw.visitLdcInsn(com.alibaba.fastjson.asm.Type.getType(desc(fieldInfo.declaringClass)));
                mw.visitLdcInsn(fieldInfo.method.getName());
                mw.visitMethodInsn(INVOKESTATIC, type(ASMUtils.class), "getMethodType",
                                   "(Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/reflect/Type;");

            } else {
                mw.visitVarInsn(ALOAD, 0);
                mw.visitLdcInsn(i);
                mw.visitMethodInsn(INVOKESPECIAL, JavaBeanSerializer, "getFieldType", "(I)Ljava/lang/reflect/Type;");
            }

            mw.visitFieldInsn(PUTFIELD, classNameType, fieldInfo.name + "_asm_fieldType", "Ljava/lang/reflect/Type;");
        }

        mw.visitInsn(RETURN);
        mw.visitMaxs(4, 4);
        mw.visitEnd();

        boolean DisableCircularReferenceDetect = false;
        if (jsonType != null) {
            for (SerializerFeature featrues : jsonType.serialzeFeatures()) {
                if (featrues == SerializerFeature.DisableCircularReferenceDetect) {
                    DisableCircularReferenceDetect = true;
                    break;
                }
            }
        }

        // 0 write
        // 1 writeNormal
        // 2 writeNonContext
        for (int i = 0; i < 3; ++i) {
            String methodName;
            boolean nonContext = DisableCircularReferenceDetect;
            boolean writeDirect = false;
            if (i == 0) {
                methodName = "write";
                writeDirect = true;
            } else if (i == 1) {
                methodName = "writeNormal";
            } else {
                writeDirect = true;
                nonContext = true;
                methodName = "writeDirectNonContext";
            }

            Context context = new Context(getters, beanInfo, classNameType, writeDirect,
                                          nonContext);

            mw = new MethodWriter(cw, //
                                  ACC_PUBLIC, //
                                  methodName, //
                                  "(L" + JSONSerializer
                                              + ";Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/reflect/Type;I)V", //
                                  null, //
                                  new String[] { "java/io/IOException" } //
            );

            mw.visitVarInsn(ALOAD, Context.serializer);
            mw.visitFieldInsn(GETFIELD, JSONSerializer, "out", SerializeWriter_desc);
            mw.visitVarInsn(ASTORE, context.var("out"));

            if ((!nativeSorted) //
                && !context.writeDirect) {

                if (jsonType == null || jsonType.alphabetic()) {
                    Label _else = new Label();

                    mw.visitVarInsn(ALOAD, context.var("out"));
                    mw.visitMethodInsn(INVOKEVIRTUAL, SerializeWriter, "isSortField", "()Z");

                    mw.visitJumpInsn(IFNE, _else);
                    mw.visitVarInsn(ALOAD, 0);
                    mw.visitVarInsn(ALOAD, 1);
                    mw.visitVarInsn(ALOAD, 2);
                    mw.visitVarInsn(ALOAD, 3);
                    mw.visitVarInsn(ALOAD, 4);
                    mw.visitVarInsn(ILOAD, 5);
                    mw.visitMethodInsn(INVOKEVIRTUAL, classNameType,
                                       "writeUnsorted", "(L" + JSONSerializer
                                                        + ";Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/reflect/Type;I)V");
                    mw.visitInsn(RETURN);

                    mw.visitLabel(_else);
                }
            }

            // isWriteDoubleQuoteDirect
            if (context.writeDirect && !nonContext) {
                Label _direct = new Label();
                Label _directElse = new Label();

                mw.visitVarInsn(ALOAD, 0);
                mw.visitVarInsn(ALOAD, Context.serializer);
                mw.visitMethodInsn(INVOKEVIRTUAL, JavaBeanSerializer, "writeDirect", "(L" + JSONSerializer + ";)Z");
                mw.visitJumpInsn(IFNE, _directElse);

                mw.visitVarInsn(ALOAD, 0);
                mw.visitVarInsn(ALOAD, 1);
                mw.visitVarInsn(ALOAD, 2);
                mw.visitVarInsn(ALOAD, 3);
                mw.visitVarInsn(ALOAD, 4);
                mw.visitVarInsn(ILOAD, 5);
                mw.visitMethodInsn(INVOKEVIRTUAL, classNameType,
                                   "writeNormal", "(L" + JSONSerializer
                                                  + ";Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/reflect/Type;I)V");
                mw.visitInsn(RETURN);

                mw.visitLabel(_directElse);
                mw.visitVarInsn(ALOAD, context.var("out"));
                mw.visitLdcInsn(SerializerFeature.DisableCircularReferenceDetect.mask);
                mw.visitMethodInsn(INVOKEVIRTUAL, SerializeWriter, "isEnabled", "(I)Z");
                mw.visitJumpInsn(IFEQ, _direct);

                mw.visitVarInsn(ALOAD, 0);
                mw.visitVarInsn(ALOAD, 1);
                mw.visitVarInsn(ALOAD, 2);
                mw.visitVarInsn(ALOAD, 3);
                mw.visitVarInsn(ALOAD, 4);
                mw.visitVarInsn(ILOAD, 5);
                mw.visitMethodInsn(INVOKEVIRTUAL, classNameType, "writeDirectNonContext",
                                   "(L" + JSONSerializer + ";Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/reflect/Type;I)V");
                mw.visitInsn(RETURN);

                mw.visitLabel(_direct);
            }

            mw.visitVarInsn(ALOAD, Context.obj); // obj
            mw.visitTypeInsn(CHECKCAST, type(clazz)); // serializer
            mw.visitVarInsn(ASTORE, context.var("entity")); // obj
            generateWriteMethod(clazz, mw, getters, context);
            mw.visitInsn(RETURN);
            mw.visitMaxs(7, context.variantIndex + 2);
            mw.visitEnd();
        }

        if (!nativeSorted) {
            // sortField support
            Context context = new Context(getters, beanInfo, classNameType, false,
                                          DisableCircularReferenceDetect);

            mw = new MethodWriter(cw, ACC_PUBLIC, "writeUnsorted",
                                  "(L" + JSONSerializer + ";Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/reflect/Type;I)V",
                                  null, new String[] { "java/io/IOException" });

            mw.visitVarInsn(ALOAD, Context.serializer);
            mw.visitFieldInsn(GETFIELD, JSONSerializer, "out", SerializeWriter_desc);
            mw.visitVarInsn(ASTORE, context.var("out"));

            mw.visitVarInsn(ALOAD, Context.obj); // obj
            mw.visitTypeInsn(CHECKCAST, type(clazz)); // serializer
            mw.visitVarInsn(ASTORE, context.var("entity")); // obj

            generateWriteMethod(clazz, mw, unsortedGetters, context);

            mw.visitInsn(RETURN);
            mw.visitMaxs(7, context.variantIndex + 2);
            mw.visitEnd();
        }

        // 0 writeAsArray
        // 1 writeAsArrayNormal
        // 2 writeAsArrayNonContext
        for (int i = 0; i < 3; ++i) {
            String methodName;
            boolean nonContext = DisableCircularReferenceDetect;
            boolean writeDirect = false;
            if (i == 0) {
                methodName = "writeAsArray";
                writeDirect = true;
            } else if (i == 1) {
                methodName = "writeAsArrayNormal";
            } else {
                writeDirect = true;
                nonContext = true;
                methodName = "writeAsArrayNonContext";
            }

            Context context = new Context(getters, beanInfo, classNameType, writeDirect,
                                          nonContext);

            mw = new MethodWriter(cw, ACC_PUBLIC, methodName,
                                  "(L" + JSONSerializer + ";Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/reflect/Type;I)V",
                                  null, new String[] { "java/io/IOException" });

            mw.visitVarInsn(ALOAD, Context.serializer);
            mw.visitFieldInsn(GETFIELD, JSONSerializer, "out", SerializeWriter_desc);
            mw.visitVarInsn(ASTORE, context.var("out"));

            mw.visitVarInsn(ALOAD, Context.obj); // obj
            mw.visitTypeInsn(CHECKCAST, type(clazz)); // serializer
            mw.visitVarInsn(ASTORE, context.var("entity")); // obj
            generateWriteAsArray(clazz, mw, getters, context);
            mw.visitInsn(RETURN);
            mw.visitMaxs(7, context.variantIndex + 2);
            mw.visitEnd();
        }

        byte[] code = cw.toByteArray();

        Class<?> exampleClass = classLoader.defineClassPublic(classNameFull, code, 0, code.length);
        Constructor<?> constructor = exampleClass.getConstructor(SerializeBeanInfo.class);
        Object instance = constructor.newInstance(beanInfo);

        return (JavaBeanSerializer) instance;
    }

再回退到上一步，查找调用链路：

// 如果没有找到，就创建一个类
    public ObjectSerializer getObjectWriter(Class<?> clazz) {
        return getObjectWriter(clazz, true);
    }

// classLoader 多次加载
    private ObjectSerializer getObjectWriter(Class<?> clazz, boolean create) {
        ObjectSerializer writer = serializers.get(clazz);

        if (writer == null) {
            try {
                final ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
                for (Object o : ServiceLoader.load(AutowiredObjectSerializer.class, classLoader)) {
                    if (!(o instanceof AutowiredObjectSerializer)) {
                        continue;
                    }

                    AutowiredObjectSerializer autowired = (AutowiredObjectSerializer) o;
                    for (Type forType : autowired.getAutowiredFor()) {
                        put(forType, autowired);
                    }
                }
            } catch (ClassCastException ex) {
                // skip
            }

            writer = serializers.get(clazz);
        }

        if (writer == null) {
            final ClassLoader classLoader = JSON.class.getClassLoader();
            if (classLoader != Thread.currentThread().getContextClassLoader()) {
                try {
                    for (Object o : ServiceLoader.load(AutowiredObjectSerializer.class, classLoader)) {

                        if (!(o instanceof AutowiredObjectSerializer)) {
                            continue;
                        }

                        AutowiredObjectSerializer autowired = (AutowiredObjectSerializer) o;
                        for (Type forType : autowired.getAutowiredFor()) {
                            put(forType, autowired);
                        }
                    }
                } catch (ClassCastException ex) {
                    // skip
                }

                writer = serializers.get(clazz);
            }
        }

        if (writer == null) {
            if (Map.class.isAssignableFrom(clazz)) {
                put(clazz, MapSerializer.instance);
            } else if (List.class.isAssignableFrom(clazz)) {
                put(clazz, ListSerializer.instance);
            } else if (Collection.class.isAssignableFrom(clazz)) {
                put(clazz, CollectionCodec.instance);
            } else if (Date.class.isAssignableFrom(clazz)) {
                put(clazz, DateCodec.instance);
            } else if (JSONAware.class.isAssignableFrom(clazz)) {
                put(clazz, JSONAwareSerializer.instance);
            } else if (JSONSerializable.class.isAssignableFrom(clazz)) {
                put(clazz, JSONSerializableSerializer.instance);
            } else if (JSONStreamAware.class.isAssignableFrom(clazz)) {
                put(clazz, MiscCodec.instance);
            } else if (clazz.isEnum() || (clazz.getSuperclass() != null && clazz.getSuperclass().isEnum())) {
                JSONType jsonType = clazz.getAnnotation(JSONType.class);
                if (jsonType != null && jsonType.serializeEnumAsJavaBean()) {
                    put(clazz, createJavaBeanSerializer(clazz));
                } else {
                    put(clazz, EnumSerializer.instance);
                }
            } else if (clazz.isArray()) {
                Class<?> componentType = clazz.getComponentType();
                ObjectSerializer compObjectSerializer = getObjectWriter(componentType);
                put(clazz, new ArraySerializer(componentType, compObjectSerializer));
            } else if (Throwable.class.isAssignableFrom(clazz)) {
                SerializeBeanInfo beanInfo = TypeUtils.buildBeanInfo(clazz, null, propertyNamingStrategy);
                beanInfo.features |= SerializerFeature.WriteClassName.mask;
                put(clazz, new JavaBeanSerializer(beanInfo));
            } else if (TimeZone.class.isAssignableFrom(clazz) || Map.Entry.class.isAssignableFrom(clazz)) {
                put(clazz, MiscCodec.instance);
            } else if (Appendable.class.isAssignableFrom(clazz)) {
                put(clazz, AppendableSerializer.instance);
            } else if (Charset.class.isAssignableFrom(clazz)) {
                put(clazz, ToStringSerializer.instance);
            } else if (Enumeration.class.isAssignableFrom(clazz)) {
                put(clazz, EnumerationSerializer.instance);
            } else if (Calendar.class.isAssignableFrom(clazz) //
                    || XMLGregorianCalendar.class.isAssignableFrom(clazz)) {
                put(clazz, CalendarCodec.instance);
            } else if (Clob.class.isAssignableFrom(clazz)) {
                put(clazz, ClobSeriliazer.instance);
            } else if (TypeUtils.isPath(clazz)) {
                put(clazz, ToStringSerializer.instance);
            } else if (Iterator.class.isAssignableFrom(clazz)) {
                put(clazz, MiscCodec.instance);
            } else {
                String className = clazz.getName();
                if (className.startsWith("java.awt.") //
                    && AwtCodec.support(clazz) //
                ) {
                    // awt
                    if (!awtError) {
                        try {
                            put(Class.forName("java.awt.Color"), AwtCodec.instance);
                            put(Class.forName("java.awt.Font"), AwtCodec.instance);
                            put(Class.forName("java.awt.Point"), AwtCodec.instance);
                            put(Class.forName("java.awt.Rectangle"), AwtCodec.instance);
                        } catch (Throwable e) {
                            awtError = true;
                            // skip
                        }
                    }
                    return  AwtCodec.instance;
                }

                // jdk8
                if ((!jdk8Error) //
                    && (className.startsWith("java.time.") //
                        || className.startsWith("java.util.Optional") //
                    )) {
                    try {
                        put(Class.forName("java.time.LocalDateTime"), Jdk8DateCodec.instance);
                        put(Class.forName("java.time.LocalDate"), Jdk8DateCodec.instance);
                        put(Class.forName("java.time.LocalTime"), Jdk8DateCodec.instance);
                        put(Class.forName("java.time.ZonedDateTime"), Jdk8DateCodec.instance);
                        put(Class.forName("java.time.OffsetDateTime"), Jdk8DateCodec.instance);
                        put(Class.forName("java.time.OffsetTime"), Jdk8DateCodec.instance);
                        put(Class.forName("java.time.ZoneOffset"), Jdk8DateCodec.instance);
                        put(Class.forName("java.time.ZoneRegion"), Jdk8DateCodec.instance);
                        put(Class.forName("java.time.Period"), Jdk8DateCodec.instance);
                        put(Class.forName("java.time.Duration"), Jdk8DateCodec.instance);
                        put(Class.forName("java.time.Instant"), Jdk8DateCodec.instance);

                        put(Class.forName("java.util.Optional"), OptionalCodec.instance);
                        put(Class.forName("java.util.OptionalDouble"), OptionalCodec.instance);
                        put(Class.forName("java.util.OptionalInt"), OptionalCodec.instance);
                        put(Class.forName("java.util.OptionalLong"), OptionalCodec.instance);

                        writer = serializers.get(clazz);
                        if (writer != null) {
                            return writer;
                        }
                    } catch (Throwable e) {
                        // skip
                        jdk8Error = true;
                    }
                }

                if ((!oracleJdbcError) //
                    && className.startsWith("oracle.sql.")) {
                    try {
                        put(Class.forName("oracle.sql.DATE"), DateCodec.instance);
                        put(Class.forName("oracle.sql.TIMESTAMP"), DateCodec.instance);

                        writer = serializers.get(clazz);
                        if (writer != null) {
                            return writer;
                        }
                    } catch (Throwable e) {
                        // skip
                        oracleJdbcError = true;
                    }
                }

                if ((!springfoxError) //
                    && className.equals("springfox.documentation.spring.web.json.Json")) {
                    try {
                        put(Class.forName("springfox.documentation.spring.web.json.Json"), //
                            SwaggerJsonSerializer.instance);

                        writer = serializers.get(clazz);
                        if (writer != null) {
                            return writer;
                        }
                    } catch (ClassNotFoundException e) {
                        // skip
                        springfoxError = true;
                    }
                }

                if ((!guavaError) //
                        && className.startsWith("com.google.common.collect.")) {
                    try {
                        put(Class.forName("com.google.common.collect.HashMultimap"), //
                                GuavaCodec.instance);
                        put(Class.forName("com.google.common.collect.LinkedListMultimap"), //
                                GuavaCodec.instance);
                        put(Class.forName("com.google.common.collect.ArrayListMultimap"), //
                                GuavaCodec.instance);
                        put(Class.forName("com.google.common.collect.TreeMultimap"), //
                                GuavaCodec.instance);

                        writer = serializers.get(clazz);
                        if (writer != null) {
                            return writer;
                        }
                    } catch (ClassNotFoundException e) {
                        // skip
                        guavaError = true;
                    }
                }

                if (className.equals("net.sf.json.JSONNull")) {
                    try {
                        put(Class.forName("net.sf.json.JSONNull"), //
                                MiscCodec.instance);
                    } catch (ClassNotFoundException e) {
                        // skip
                    }
                    writer = serializers.get(clazz);
                    if (writer != null) {
                        return writer;
                    }
                }

                if (TypeUtils.isProxy(clazz)) {
                    Class<?> superClazz = clazz.getSuperclass();

                    ObjectSerializer superWriter = getObjectWriter(superClazz);
                    put(clazz, superWriter);
                    return superWriter;
                }

                // asm创建类是在这里生效的
                if (create) {
                    put(clazz, createJavaBeanSerializer(clazz));
                }
            }

            writer = serializers.get(clazz);
        }
        return writer;
    }

往前追踪，发现入口：

// TypeUtils, 获取所有get方法。
    public static List<FieldInfo> computeGetters(Class<?> clazz, //
                                                 JSONType jsonType, //
                                                 Map<String, String> aliasMap, //
                                                 Map<String, Field> fieldCacheMap, //
                                                 boolean sorted, //
                                                 PropertyNamingStrategy propertyNamingStrategy //
    ) {
        Map<String, FieldInfo> fieldInfoMap = new LinkedHashMap<String, FieldInfo>();

        for (Method method : clazz.getMethods()) {
            String methodName = method.getName();
            int ordinal = 0, serialzeFeatures = 0, parserFeatures = 0;
            String label = null;

            if (Modifier.isStatic(method.getModifiers())) {
                continue;
            }

            if (method.getReturnType().equals(Void.TYPE)) {
                continue;
            }

            if (method.getParameterTypes().length != 0) {
                continue;
            }

            if (method.getReturnType() == ClassLoader.class) {
                continue;
            }

            if (method.getName().equals("getMetaClass")
                && method.getReturnType().getName().equals("groovy.lang.MetaClass")) {
                continue;
            }

            JSONField annotation = method.getAnnotation(JSONField.class);

            if (annotation == null) {
                annotation = getSuperMethodAnnotation(clazz, method);
            }

            if (annotation != null) {
                if (!annotation.serialize()) {
                    continue;
                }

                ordinal = annotation.ordinal();
                serialzeFeatures = SerializerFeature.of(annotation.serialzeFeatures());
                parserFeatures = Feature.of(annotation.parseFeatures());

                if (annotation.name().length() != 0) {
                    String propertyName = annotation.name();

                    if (aliasMap != null) {
                        propertyName = aliasMap.get(propertyName);
                        if (propertyName == null) {
                            continue;
                        }
                    }

                    FieldInfo fieldInfo = new FieldInfo(propertyName, method, null, clazz, null, ordinal,
                                                        serialzeFeatures, parserFeatures, annotation, null, label);
                    fieldInfoMap.put(propertyName, fieldInfo);
                    continue;
                }

                if (annotation.label().length() != 0) {
                    label = annotation.label();
                }
            }

            if (methodName.startsWith("get")) {
                if (methodName.length() < 4) {
                    continue;
                }

                if (methodName.equals("getClass")) {
                    continue;
                }

                if (methodName.equals("getDeclaringClass") && clazz.isEnum()) {
                    continue;
                }

                char c3 = methodName.charAt(3);

                String propertyName;
                if (Character.isUpperCase(c3) //
                    || c3 > 512 // for unicode method name
                ) {
                   if (compatibleWithJavaBean) {
                        propertyName = decapitalize(methodName.substring(3));
                    } else {
                        propertyName = Character.toLowerCase(methodName.charAt(3)) + methodName.substring(4);
                    }
                    propertyName = getPropertyNameByCompatibleFieldName(fieldCacheMap, methodName,  propertyName,3);
                } else if (c3 == '_') {
                    propertyName = methodName.substring(4);
                } else if (c3 == 'f') {
                    propertyName = methodName.substring(3);
                } else if (methodName.length() >= 5 && Character.isUpperCase(methodName.charAt(4))) {
                    propertyName = decapitalize(methodName.substring(3));
                } else {
                    continue;
                }

                boolean ignore = isJSONTypeIgnore(clazz, propertyName);

                if (ignore) {
                    continue;
                }
                //假如bean的field很多的情况一下，轮询时将大大降低效率
                Field field = ParserConfig.getFieldFromCache(propertyName, fieldCacheMap);

                if (field == null && propertyName.length() > 1) {
                    char ch = propertyName.charAt(1);
                    if (ch >= 'A' && ch <= 'Z') {
                        String javaBeanCompatiblePropertyName = decapitalize(methodName.substring(3));
                        field = ParserConfig.getFieldFromCache(javaBeanCompatiblePropertyName, fieldCacheMap);
                    }
                }

                JSONField fieldAnnotation = null;
                if (field != null) {
                    fieldAnnotation = field.getAnnotation(JSONField.class);

                    if (fieldAnnotation != null) {
                        if (!fieldAnnotation.serialize()) {
                            continue;
                        }

                        ordinal = fieldAnnotation.ordinal();
                        serialzeFeatures = SerializerFeature.of(fieldAnnotation.serialzeFeatures());
                        parserFeatures = Feature.of(fieldAnnotation.parseFeatures());

                        if (fieldAnnotation.name().length() != 0) {
                            propertyName = fieldAnnotation.name();

                            if (aliasMap != null) {
                                propertyName = aliasMap.get(propertyName);
                                if (propertyName == null) {
                                    continue;
                                }
                            }
                        }

                        if (fieldAnnotation.label().length() != 0) {
                            label = fieldAnnotation.label();
                        }
                    }
                }

                if (aliasMap != null) {
                    propertyName = aliasMap.get(propertyName);
                    if (propertyName == null) {
                        continue;
                    }
                }

                if (propertyNamingStrategy != null) {
                    propertyName = propertyNamingStrategy.translate(propertyName);
                }

                FieldInfo fieldInfo = new FieldInfo(propertyName, method, field, clazz, null, ordinal, serialzeFeatures, parserFeatures,
                                                    annotation, fieldAnnotation, label);
                fieldInfoMap.put(propertyName, fieldInfo);
            }

            if (methodName.startsWith("is")) {
                if (methodName.length() < 3) {
                    continue;
                }

                if (method.getReturnType() != Boolean.TYPE
                        && method.getReturnType() != Boolean.class) {
                    continue;
                }

                char c2 = methodName.charAt(2);

                String propertyName;
                if (Character.isUpperCase(c2)) {
                    if (compatibleWithJavaBean) {
                        propertyName = decapitalize(methodName.substring(2));
                    } else {
                        propertyName = Character.toLowerCase(methodName.charAt(2)) + methodName.substring(3);
                    }
                    propertyName = getPropertyNameByCompatibleFieldName(fieldCacheMap, methodName,  propertyName,2);
                } else if (c2 == '_') {
                    propertyName = methodName.substring(3);
                } else if (c2 == 'f') {
                    propertyName = methodName.substring(2);
                } else {
                    continue;
                }

                Field field = ParserConfig.getFieldFromCache(propertyName,fieldCacheMap);

                if (field == null) {
                    field = ParserConfig.getFieldFromCache(methodName,fieldCacheMap);
                }

                JSONField fieldAnnotation = null;
                if (field != null) {
                    fieldAnnotation = field.getAnnotation(JSONField.class);

                    if (fieldAnnotation != null) {
                        if (!fieldAnnotation.serialize()) {
                            continue;
                        }

                        ordinal = fieldAnnotation.ordinal();
                        serialzeFeatures = SerializerFeature.of(fieldAnnotation.serialzeFeatures());
                        parserFeatures = Feature.of(fieldAnnotation.parseFeatures());

                        if (fieldAnnotation.name().length() != 0) {
                            propertyName = fieldAnnotation.name();

                            if (aliasMap != null) {
                                propertyName = aliasMap.get(propertyName);
                                if (propertyName == null) {
                                    continue;
                                }
                            }
                        }

                        if (fieldAnnotation.label().length() != 0) {
                            label = fieldAnnotation.label();
                        }
                    }
                }

                if (aliasMap != null) {
                    propertyName = aliasMap.get(propertyName);
                    if (propertyName == null) {
                        continue;
                    }
                }

                if (propertyNamingStrategy != null) {
                    propertyName = propertyNamingStrategy.translate(propertyName);
                }

                //优先选择get
                if (fieldInfoMap.containsKey(propertyName)) {
                    continue;
                }

                FieldInfo fieldInfo = new FieldInfo(propertyName, method, field, clazz, null, ordinal, serialzeFeatures, parserFeatures,
                                                    annotation, fieldAnnotation, label);
                fieldInfoMap.put(propertyName, fieldInfo);
            }
        }

        for (Field field : clazz.getFields()) {
            if (Modifier.isStatic(field.getModifiers())) {
                continue;
            }

            JSONField fieldAnnotation = field.getAnnotation(JSONField.class);

            int ordinal = 0, serialzeFeatures = 0, parserFeatures = 0;
            String propertyName = field.getName();
            String label = null;
            if (fieldAnnotation != null) {
                if (!fieldAnnotation.serialize()) {
                    continue;
                }

                ordinal = fieldAnnotation.ordinal();
                serialzeFeatures = SerializerFeature.of(fieldAnnotation.serialzeFeatures());
                parserFeatures = Feature.of(fieldAnnotation.parseFeatures());

                if (fieldAnnotation.name().length() != 0) {
                    propertyName = fieldAnnotation.name();
                }

                if (fieldAnnotation.label().length() != 0) {
                    label = fieldAnnotation.label();
                }
            }

            if (aliasMap != null) {
                propertyName = aliasMap.get(propertyName);
                if (propertyName == null) {
                    continue;
                }
            }

            if (propertyNamingStrategy != null) {
                propertyName = propertyNamingStrategy.translate(propertyName);
            }

            if (!fieldInfoMap.containsKey(propertyName)) {
                FieldInfo fieldInfo = new FieldInfo(propertyName, null, field, clazz, null, ordinal, serialzeFeatures, parserFeatures,
                                                    null, fieldAnnotation, label);
                fieldInfoMap.put(propertyName, fieldInfo);
            }
        }

        List<FieldInfo> fieldInfoList = new ArrayList<FieldInfo>();

        boolean containsAll = false;
        String[] orders = null;

        JSONType annotation = clazz.getAnnotation(JSONType.class);
        if (annotation != null) {
            orders = annotation.orders();

            if (orders != null && orders.length == fieldInfoMap.size()) {
                containsAll = true;
                for (String item : orders) {
                    if (!fieldInfoMap.containsKey(item)) {
                        containsAll = false;
                        break;
                    }
                }
            } else {
                containsAll = false;
            }
        }

        if (containsAll) {
            for (String item : orders) {
                FieldInfo fieldInfo = fieldInfoMap.get(item);
                fieldInfoList.add(fieldInfo);
            }
        } else {
            for (FieldInfo fieldInfo : fieldInfoMap.values()) {
                fieldInfoList.add(fieldInfo);
            }

            if (sorted) {
                Collections.sort(fieldInfoList);
            }
        }

        return fieldInfoList;
    }

// SerializeConfig, 创建java bean.

    public ObjectSerializer createJavaBeanSerializer(SerializeBeanInfo beanInfo) {
        JSONType jsonType = beanInfo.jsonType;

        if (jsonType != null) {
            Class<?> serializerClass = jsonType.serializer();
            if (serializerClass != Void.class) {
                try {
                    Object seralizer = serializerClass.newInstance();
                    if (seralizer instanceof ObjectSerializer) {
                        return (ObjectSerializer) seralizer;
                    }
                } catch (Throwable e) {
                    // skip
                }
            }

            if (jsonType.asm() == false) {
                asm = false;
            }
        }

        Class<?> clazz = beanInfo.beanType;
        if (!Modifier.isPublic(beanInfo.beanType.getModifiers())) {
            return new JavaBeanSerializer(beanInfo);
        }

        boolean asm = this.asm;

        if (asm && asmFactory.classLoader.isExternalClass(clazz)
                || clazz == Serializable.class || clazz == Object.class) {
            asm = false;
        }

        if (asm && !ASMUtils.checkName(clazz.getSimpleName())) {
            asm = false;
        }

        if (asm) {
            for(FieldInfo fieldInfo : beanInfo.fields){
                Field field = fieldInfo.field;
                if (field != null && !field.getType().equals(fieldInfo.fieldClass)) {
                    asm = false;
                    break;
                }

                Method method = fieldInfo.method;
                if (method != null && !method.getReturnType().equals(fieldInfo.fieldClass)) {
                    asm = false;
                    break;
                }

                JSONField annotation = fieldInfo.getAnnotation();

                if (annotation == null) {
                    continue;
                }
                if ((!ASMUtils.checkName(annotation.name())) //
                        || annotation.format().length() != 0
                        || annotation.jsonDirect()
                        || annotation.serializeUsing() != Void.class
                        ) {
                    asm = false;
                    break;
                }
            }
        }

        if (asm) {
            try {
                ObjectSerializer asmSerializer = createASMSerializer(beanInfo);
                if (asmSerializer != null) {
                    return asmSerializer;
                }
            } catch (ClassFormatError e) {
                // skip
            } catch (ClassCastException e) {
                // skip
            } catch (Throwable e) {
                throw new JSONException("create asm serializer error, class "
                        + clazz, e);
            }
        }

        return new JavaBeanSerializer(beanInfo);
    }

使用asmClassLoader加载动态类：

// 创建asmSerializer, 最后调用classloader加载类
    private final JavaBeanSerializer createASMSerializer(SerializeBeanInfo beanInfo) throws Exception {
        JavaBeanSerializer serializer = asmFactory.createJavaBeanSerializer(beanInfo);

        for (int i = 0; i < serializer.sortedGetters.length; ++i) {
            FieldSerializer fieldDeser = serializer.sortedGetters[i];
            Class<?> fieldClass = fieldDeser.fieldInfo.fieldClass;
            if (fieldClass.isEnum()) {
                ObjectSerializer fieldSer = this.getObjectWriter(fieldClass);
                if (!(fieldSer instanceof EnumSerializer)) {
                    serializer.writeDirect = false;
                }
            }
        }

        return serializer;
    }

　　最后的代码报错是：java.lang.IllegalStateException: It is illegal to call this method if the current request is not in asynchronous mode (i.e. isAsyncStarted() returns false)，由于是写在动态生成的代码文件中，比较难查，解决问题已足够，不再深究。

　　理解这里的原因后，再来看问题，就简单多了，排除掉不支持转换的类或者使用其他方式获取想要的参数。
　　思考：如果让自己来设计框架，也需要考虑得更多；在用别人的框架的时候，也以一个开发者的角度考虑问题，如果不想考虑，可以直接参考其源码，发现其中的蹊跷；另外，测试一定是必不可少的，你可以借助测试部门，也可以自行测试，但是一定要考虑全面；最后，在做一些公用方法的变动时，一定不要太大意，宁可相信自己不无知，也不要相信代码一定不会出错，有条件情况下，尽可以使用try catch将其包裹，这样，在出问题的时候，也能迅速定位。
　　题外话：错误一定是会犯的，但是不要关键时候犯。