深入分析Java反射(七)-简述反射调用的底层实现

前提

Java反射的API在JavaSE1.7的时候已经基本完善，但是本文编写的时候使用的是Oracle JDK11，因为JDK11对于sun包下的源码也上传了，可以直接通过IDE查看对应的源码和进行Debug。

本文主要介绍反射调用的底层实现，当然还没有能力分析JVM的实现，这里只分析到最终Native方法的调用点。底层会依赖到Unsafe类，可以的话可以看下笔者之前写的一篇文章《神奇的魔法类和双刃剑-Unsafe》。

反射调用的底层实现探究

主要考虑下面的情况：

• 属性操作：java.lang.reflect.Field#set(Object obj, Object value)和java.lang.reflect.Field#get(Object obj)。
• 构造器调用：java.lang.reflect.Constructor#newInstance(Object ... initargs)。
• 方法调用：java.lang.reflect.Method#invoke(Object obj, Object... args)。

处理属性操作的底层实现

属性操作方法Field#set(Object obj, Object value)和Field#get(Object obj)底层都是委托到jdk.internal.reflect.FieldAccessor实现：

public interface FieldAccessor {
    /** Matches specification in {@link java.lang.reflect.Field} */
    public Object get(Object obj) throws IllegalArgumentException;

    /** Matches specification in {@link java.lang.reflect.Field} */
    public boolean getBoolean(Object obj) throws IllegalArgumentException;

    /** Matches specification in {@link java.lang.reflect.Field} */
    public byte getByte(Object obj) throws IllegalArgumentException;

    /** Matches specification in {@link java.lang.reflect.Field} */
    public char getChar(Object obj) throws IllegalArgumentException;

    /** Matches specification in {@link java.lang.reflect.Field} */
    public short getShort(Object obj) throws IllegalArgumentException;

    /** Matches specification in {@link java.lang.reflect.Field} */
    public int getInt(Object obj) throws IllegalArgumentException;

    /** Matches specification in {@link java.lang.reflect.Field} */
    public long getLong(Object obj) throws IllegalArgumentException;

    /** Matches specification in {@link java.lang.reflect.Field} */
    public float getFloat(Object obj) throws IllegalArgumentException;

    /** Matches specification in {@link java.lang.reflect.Field} */
    public double getDouble(Object obj) throws IllegalArgumentException;

    /** Matches specification in {@link java.lang.reflect.Field} */
    public void set(Object obj, Object value)
        throws IllegalArgumentException, IllegalAccessException;

    /** Matches specification in {@link java.lang.reflect.Field} */
    public void setBoolean(Object obj, boolean z)
        throws IllegalArgumentException, IllegalAccessException;

    /** Matches specification in {@link java.lang.reflect.Field} */
    public void setByte(Object obj, byte b)
        throws IllegalArgumentException, IllegalAccessException;

    /** Matches specification in {@link java.lang.reflect.Field} */
    public void setChar(Object obj, char c)
        throws IllegalArgumentException, IllegalAccessException;

    /** Matches specification in {@link java.lang.reflect.Field} */
    public void setShort(Object obj, short s)
        throws IllegalArgumentException, IllegalAccessException;

    /** Matches specification in {@link java.lang.reflect.Field} */
    public void setInt(Object obj, int i)
        throws IllegalArgumentException, IllegalAccessException;

    /** Matches specification in {@link java.lang.reflect.Field} */
    public void setLong(Object obj, long l)
        throws IllegalArgumentException, IllegalAccessException;

    /** Matches specification in {@link java.lang.reflect.Field} */
    public void setFloat(Object obj, float f)
        throws IllegalArgumentException, IllegalAccessException;

    /** Matches specification in {@link java.lang.reflect.Field} */
    public void setDouble(Object obj, double d)
        throws IllegalArgumentException, IllegalAccessException;
}

FieldAccessor接口有很多的实现，FieldAccessor接口实例是通过jdk.internal.reflect.ReflectionFactory这个工厂构造的：

    public FieldAccessor newFieldAccessor(Field field, boolean override) {
        checkInitted();

        Field root = langReflectAccess.getRoot(field);
        if (root != null) {
            // FieldAccessor will use the root unless the modifiers have
            // been overrridden
            if (root.getModifiers() == field.getModifiers() || !override) {
                field = root;
            }
        }
        return UnsafeFieldAccessorFactory.newFieldAccessor(field, override);
    }

最终委托到UnsafeFieldAccessorFactory#newFieldAccessor()：

class UnsafeFieldAccessorFactory {
    static FieldAccessor newFieldAccessor(Field field, boolean override) {
        Class<?> type = field.getType();
        boolean isStatic = Modifier.isStatic(field.getModifiers());
        boolean isFinal = Modifier.isFinal(field.getModifiers());
        boolean isVolatile = Modifier.isVolatile(field.getModifiers());
        boolean isQualified = isFinal || isVolatile;
        boolean isReadOnly = isFinal && (isStatic || !override);
        if (isStatic) {
            // This code path does not guarantee that the field's
            // declaring class has been initialized, but it must be
            // before performing reflective operations.
            UnsafeFieldAccessorImpl.unsafe.ensureClassInitialized(field.getDeclaringClass());

            if (!isQualified) {
                if (type == Boolean.TYPE) {
                    return new UnsafeStaticBooleanFieldAccessorImpl(field);
                } else if (type == Byte.TYPE) {
                    return new UnsafeStaticByteFieldAccessorImpl(field);
                } else if (type == Short.TYPE) {
                    return new UnsafeStaticShortFieldAccessorImpl(field);
                } else if (type == Character.TYPE) {
                    return new UnsafeStaticCharacterFieldAccessorImpl(field);
                } else if (type == Integer.TYPE) {
                    return new UnsafeStaticIntegerFieldAccessorImpl(field);
                } else if (type == Long.TYPE) {
                    return new UnsafeStaticLongFieldAccessorImpl(field);
                } else if (type == Float.TYPE) {
                    return new UnsafeStaticFloatFieldAccessorImpl(field);
                } else if (type == Double.TYPE) {
                    return new UnsafeStaticDoubleFieldAccessorImpl(field);
                } else {
                    return new UnsafeStaticObjectFieldAccessorImpl(field);
                }
            } else {
                if (type == Boolean.TYPE) {
                    return new UnsafeQualifiedStaticBooleanFieldAccessorImpl(field, isReadOnly);
                } else if (type == Byte.TYPE) {
                    return new UnsafeQualifiedStaticByteFieldAccessorImpl(field, isReadOnly);
                } else if (type == Short.TYPE) {
                    return new UnsafeQualifiedStaticShortFieldAccessorImpl(field, isReadOnly);
                } else if (type == Character.TYPE) {
                    return new UnsafeQualifiedStaticCharacterFieldAccessorImpl(field, isReadOnly);
                } else if (type == Integer.TYPE) {
                    return new UnsafeQualifiedStaticIntegerFieldAccessorImpl(field, isReadOnly);
                } else if (type == Long.TYPE) {
                    return new UnsafeQualifiedStaticLongFieldAccessorImpl(field, isReadOnly);
                } else if (type == Float.TYPE) {
                    return new UnsafeQualifiedStaticFloatFieldAccessorImpl(field, isReadOnly);
                } else if (type == Double.TYPE) {
                    return new UnsafeQualifiedStaticDoubleFieldAccessorImpl(field, isReadOnly);
                } else {
                    return new UnsafeQualifiedStaticObjectFieldAccessorImpl(field, isReadOnly);
                }
            }
        } else {
            if (!isQualified) {
                if (type == Boolean.TYPE) {
                    return new UnsafeBooleanFieldAccessorImpl(field);
                } else if (type == Byte.TYPE) {
                    return new UnsafeByteFieldAccessorImpl(field);
                } else if (type == Short.TYPE) {
                    return new UnsafeShortFieldAccessorImpl(field);
                } else if (type == Character.TYPE) {
                    return new UnsafeCharacterFieldAccessorImpl(field);
                } else if (type == Integer.TYPE) {
                    return new UnsafeIntegerFieldAccessorImpl(field);
                } else if (type == Long.TYPE) {
                    return new UnsafeLongFieldAccessorImpl(field);
                } else if (type == Float.TYPE) {
                    return new UnsafeFloatFieldAccessorImpl(field);
                } else if (type == Double.TYPE) {
                    return new UnsafeDoubleFieldAccessorImpl(field);
                } else {
                    return new UnsafeObjectFieldAccessorImpl(field);
                }
            } else {
                if (type == Boolean.TYPE) {
                    return new UnsafeQualifiedBooleanFieldAccessorImpl(field, isReadOnly);
                } else if (type == Byte.TYPE) {
                    return new UnsafeQualifiedByteFieldAccessorImpl(field, isReadOnly);
                } else if (type == Short.TYPE) {
                    return new UnsafeQualifiedShortFieldAccessorImpl(field, isReadOnly);
                } else if (type == Character.TYPE) {
                    return new UnsafeQualifiedCharacterFieldAccessorImpl(field, isReadOnly);
                } else if (type == Integer.TYPE) {
                    return new UnsafeQualifiedIntegerFieldAccessorImpl(field, isReadOnly);
                } else if (type == Long.TYPE) {
                    return new UnsafeQualifiedLongFieldAccessorImpl(field, isReadOnly);
                } else if (type == Float.TYPE) {
                    return new UnsafeQualifiedFloatFieldAccessorImpl(field, isReadOnly);
                } else if (type == Double.TYPE) {
                    return new UnsafeQualifiedDoubleFieldAccessorImpl(field, isReadOnly);
                } else {
                    return new UnsafeQualifiedObjectFieldAccessorImpl(field, isReadOnly);
                }
            }
        }
    }
}

这里注意一下属性修饰符的判断：

• isStatic：静态属性，也就是static关键字修饰的属性。
• isFinal：final关键字修饰的属性。
• isVolatile：valatile关键字修饰的属性。
• isQualified：valatile关键字或者final关键字修饰的属性。
• isReadOnly：是否只读属性，final关键字修饰的属性或者static关键字修饰并且不能覆盖(override = false)的属性。

通过上面修饰符做判断，得到最终的FieldAccessor实现。这里挑一个例子进行分析，例如一个普通非静态没有volatile和final关键字修饰属性最终就会得到UnsafeObjectFieldAccessorImpl的实例：

class UnsafeObjectFieldAccessorImpl extends UnsafeFieldAccessorImpl {
    UnsafeObjectFieldAccessorImpl(Field field) {
        super(field);
    }

    public Object get(Object obj) throws IllegalArgumentException {
        ensureObj(obj);
        return unsafe.getObject(obj, fieldOffset);
    }

    public void set(Object obj, Object value)
        throws IllegalArgumentException, IllegalAccessException{
        ensureObj(obj);
        if (isFinal) {
            throwFinalFieldIllegalAccessException(value);
        }
        if (value != null) {
            if (!field.getType().isAssignableFrom(value.getClass())) {
                throwSetIllegalArgumentException(value);
            }
        }
        unsafe.putObject(obj, fieldOffset, value);
    }  

    public boolean getBoolean(Object obj) throws IllegalArgumentException {
        throw newGetBooleanIllegalArgumentException();
    }

    public byte getByte(Object obj) throws IllegalArgumentException {
        throw newGetByteIllegalArgumentException();
    }     

    // 省略其他直接抛出异常的方法
}

可见UnsafeObjectFieldAccessorImpl中除了get(Object obj)和set(Object obj, Object value)方法，其他方法都是直接抛出IllegalArgumentException。而get(Object obj)和set(Object obj, Object value)底层分别依赖于jdk.internal.misc.Unsafe的putObject(obj, fieldOffset, value)和getObject(obj, fieldOffset)方法。而属性的内存偏移地址是在UnsafeObjectFieldAccessorImpl的父类UnsafeFieldAccessorImpl的构造函数中计算出来的：

abstract class UnsafeFieldAccessorImpl extends FieldAccessorImpl {
    static final Unsafe unsafe = Unsafe.getUnsafe();

    protected final Field   field;
    protected final long    fieldOffset;
    protected final boolean isFinal;

    UnsafeFieldAccessorImpl(Field field) {
        this.field = field;
        if (Modifier.isStatic(field.getModifiers()))
            fieldOffset = unsafe.staticFieldOffset(field);
        else
            fieldOffset = unsafe.objectFieldOffset(field);
        isFinal = Modifier.isFinal(field.getModifiers());
    }
    // 省略其他方法
}

这里可以做个小结，属性反射操作Field的setXX和getXX方法最终委托到jdk.internal.misc.Unsafe的putXX和getXX方法，而属性的内存偏移地址是通过jdk.internal.misc.Unsafe的staticFieldBase()、staticFieldOffset和objectFieldOffset几个方法计算的。

处理构造器调用的底层实现

Constructor#newInstance()方法调用依赖到ConstructorAccessor：

    public T newInstance(Object ... initargs)
        throws InstantiationException, IllegalAccessException,
               IllegalArgumentException, InvocationTargetException
    {
        if (!override) {
            Class<?> caller = Reflection.getCallerClass();
            checkAccess(caller, clazz, clazz, modifiers);
        }
        if ((clazz.getModifiers() & Modifier.ENUM) != 0)
            throw new IllegalArgumentException("Cannot reflectively create enum objects");
        ConstructorAccessor ca = constructorAccessor;   // read volatile
        if (ca == null) {
            ca = acquireConstructorAccessor();
        }
        @SuppressWarnings("unchecked")
        T inst = (T) ca.newInstance(initargs);
        return inst;
    }

// ConstructorAccessor接口
public interface ConstructorAccessor {
    /** Matches specification in {@link java.lang.reflect.Constructor} */
    public Object newInstance(Object[] args)
        throws InstantiationException,
               IllegalArgumentException,
               InvocationTargetException;
}

而获取ConstructorAccessor实例也是通过反射工厂类ReflectionFactory，具体是ReflectionFactory#newConstructorAccessor：

    public ConstructorAccessor newConstructorAccessor(Constructor<?> c) {
        checkInitted();
        Class<?> declaringClass = c.getDeclaringClass();
        // 抽象方法会进入此if分支
        if (Modifier.isAbstract(declaringClass.getModifiers())) {
            return new InstantiationExceptionConstructorAccessorImpl(null);
        }
        // 宿主类直接是Class类型，则无法实例化
        if (declaringClass == Class.class) {
            return new InstantiationExceptionConstructorAccessorImpl
                ("Can not instantiate java.lang.Class");
        }

        // use the root Constructor that will not cache caller class
        Constructor<?> root = langReflectAccess.getRoot(c);
        if (root != null) {
            c = root;
        }

        // 当前声明构造的宿主类是ConstructorAccessorImpl的子类
        if (Reflection.isSubclassOf(declaringClass,
                                    ConstructorAccessorImpl.class)) {
            return new BootstrapConstructorAccessorImpl(c);
        }
        //
        if (noInflation && !ReflectUtil.isVMAnonymousClass(c.getDeclaringClass())) {
            return new MethodAccessorGenerator().
                generateConstructor(c.getDeclaringClass(),
                                    c.getParameterTypes(),
                                    c.getExceptionTypes(),
                                    c.getModifiers());
        } else {
            NativeConstructorAccessorImpl acc =
                new NativeConstructorAccessorImpl(c);
            DelegatingConstructorAccessorImpl res =
                new DelegatingConstructorAccessorImpl(acc);
            acc.setParent(res);
            return res;
        }
    }

可见最终得到的ConstructorAccessor实例为DelegatingConstructorAccessorImpl，而DelegatingConstructorAccessorImpl只是一个委托实现，底层是调用NativeConstructorAccessorImpl：

class NativeConstructorAccessorImpl extends ConstructorAccessorImpl {
    private final Constructor<?> c;
    private DelegatingConstructorAccessorImpl parent;
    private int numInvocations;

    NativeConstructorAccessorImpl(Constructor<?> c) {
        this.c = c;
    }

    public Object newInstance(Object[] args)
        throws InstantiationException,
               IllegalArgumentException,
               InvocationTargetException
    {
        // We can't inflate a constructor belonging to a vm-anonymous class
        // because that kind of class can't be referred to by name, hence can't
        // be found from the generated bytecode.
        if (++numInvocations > ReflectionFactory.inflationThreshold()
                && !ReflectUtil.isVMAnonymousClass(c.getDeclaringClass())) {
            ConstructorAccessorImpl acc = (ConstructorAccessorImpl)
                new MethodAccessorGenerator().
                    generateConstructor(c.getDeclaringClass(),
                                        c.getParameterTypes(),
                                        c.getExceptionTypes(),
                                        c.getModifiers());
            parent.setDelegate(acc);
        }

        return newInstance0(c, args);
    }

    void setParent(DelegatingConstructorAccessorImpl parent) {
        this.parent = parent;
    }
    // 这个就是最终构造实例化对象的native方法
    private static native Object newInstance0(Constructor<?> c, Object[] args)
        throws InstantiationException,
               IllegalArgumentException,
               InvocationTargetException;
}

NativeConstructorAccessorImpl#newInstance0()就是最终构造实例化对象的Native方法。当然有例外的情况，例如非正常调用下，如果构造器的宿主类是一个抽象类，那么最终会返回一个InstantiationExceptionConstructorAccessorImpl实例，里面直接抛出InstantiationException异常。

处理方法调用的底层实现

Method#invoke()调用依赖于MethodAccessor：

// MethodAccessor接口
public interface MethodAccessor {
    /** Matches specification in {@link java.lang.reflect.Method} */
    public Object invoke(Object obj, Object[] args)
        throws IllegalArgumentException, InvocationTargetException;
}

    public Object invoke(Object obj, Object... args)
        throws IllegalAccessException, IllegalArgumentException,
           InvocationTargetException{
        if (!override) {
            Class<?> caller = Reflection.getCallerClass();
            checkAccess(caller, clazz,
                        Modifier.isStatic(modifiers) ? null : obj.getClass(),
                        modifiers);
        }
        MethodAccessor ma = methodAccessor;             // read volatile
        if (ma == null) {
            ma = acquireMethodAccessor();
        }
        return ma.invoke(obj, args);
    }

获取MethodAccessor实例的逻辑和前两节类似，是通过ReflectionFactory#newMethodAccessor()：

    public MethodAccessor newMethodAccessor(Method method) {
        checkInitted();

        if (Reflection.isCallerSensitive(method)) {
            Method altMethod = findMethodForReflection(method);
            if (altMethod != null) {
                method = altMethod;
            }
        }

        // use the root Method that will not cache caller class
        Method root = langReflectAccess.getRoot(method);
        if (root != null) {
            method = root;
        }

        if (noInflation && !ReflectUtil.isVMAnonymousClass(method.getDeclaringClass())) {
            return new MethodAccessorGenerator().
                generateMethod(method.getDeclaringClass(),
                               method.getName(),
                               method.getParameterTypes(),
                               method.getReturnType(),
                               method.getExceptionTypes(),
                               method.getModifiers());
        } else {
            NativeMethodAccessorImpl acc =
                new NativeMethodAccessorImpl(method);
            DelegatingMethodAccessorImpl res =
                new DelegatingMethodAccessorImpl(acc);
            acc.setParent(res);
            return res;
        }
    }

最终会委托到NativeMethodAccessorImpl#invoke(Object obj, Object[] args)：

class NativeMethodAccessorImpl extends MethodAccessorImpl {
    private final Method method;
    private DelegatingMethodAccessorImpl parent;
    private int numInvocations;

    NativeMethodAccessorImpl(Method method) {
        this.method = method;
    }

    public Object invoke(Object obj, Object[] args)
        throws IllegalArgumentException, InvocationTargetException
    {
        // We can't inflate methods belonging to vm-anonymous classes because
        // that kind of class can't be referred to by name, hence can't be
        // found from the generated bytecode.
        if (++numInvocations > ReflectionFactory.inflationThreshold()
                && !ReflectUtil.isVMAnonymousClass(method.getDeclaringClass())) {
            MethodAccessorImpl acc = (MethodAccessorImpl)
                new MethodAccessorGenerator().
                    generateMethod(method.getDeclaringClass(),
                                   method.getName(),
                                   method.getParameterTypes(),
                                   method.getReturnType(),
                                   method.getExceptionTypes(),
                                   method.getModifiers());
            parent.setDelegate(acc);
        }

        return invoke0(method, obj, args);
    }

    void setParent(DelegatingMethodAccessorImpl parent) {
        this.parent = parent;
    }

    private static native Object invoke0(Method m, Object obj, Object[] args);
}

而NativeMethodAccessorImpl#invoke0()就是方法调用的最终调用的Native方法。

小结

学习知识过程总是阶梯式上升的，JDK中的类库设计也类似这样，如果提前熟悉Unsafe类的相关方法，其实反射调用的底层实现也能够相对轻易地理解。属性、构造和方法反射调用底层的实现(只考虑正常调用的情况下)如下：

• 对于属性(Field)：Field#setXX()和Field#getXX()分别对应Unsafe的putXX()和getXX()方法，也就是说完全依赖Unsafe中的Native方法。
• 对于构造(Constructor)：Constructor#newInstance()底层调用NativeConstructorAccessorImpl#newInstance0()。
• 对于方法(Method)：Method#invoke()底层调用NativeMethodAccessorImpl#invoke0()

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(本文完 e-a-20181216 c-1-d)