HibernateProxy异常处理 java.lang.UnsupportedOperationException: Attempted to serialize java.lang.Class: org.hibernate.proxy.HibernateProxy. Forgot to register a type adapter?

这里使用google的Gson包做JSON转换，因为较早的1.4版本的FieldAttributes类中没有getDeclaringClass()这个方法，这个方法是获取field所属的类，在我的排除策略中会用到。

排除策略 
  最简单的gson转换可以是这样的，但却没有多少实际的作用。切面日志时，一个实体和其他实体存在关联，这时候就需要通过自定义排除策略决定如何转换关联对象，否则可能出现“爆炸式”的json字符串。

Gson gson = new Gson();
int[] ints = {1, 2, 3, 4, 5};
String[] strings = {"abc", "def", "ghi"};

// Serialization
gson.toJson(ints);     ==> prints [1,2,3,4,5]
gson.toJson(strings);  ==> prints ["abc", "def", "ghi"]

下面是我定义的一个排除策略的类，能基本满足需求，从内网搬过来的，未测试

package com.lingceng.magic.logutil;

import org.apache.commons.lang.ArrayUtils;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import com.google.gson.ExclusionStrategy;
import com.google.gson.FieldAttributes;

public class TargetStrategy implements ExclusionStrategy {
    private static Logger log = LoggerFactory.getLogger(TargetStrategy.class);
    private Class<?> target;
    private String[] fields;
    private Class<?>[] clazz;
    private boolean reverse;

    public TargetStrategy(Class<?> target) {
        super();
        this.target = target;
    }

    @Override
    public boolean shouldSkipClass(Class<?> class1) {
        return false;
    }

    @Override
    public boolean shouldSkipField(FieldAttributes fieldattributes) {
        Class<?> owner = fieldattributes.getDeclaringClass();
        Class<?> c = fieldattributes.getDeclaredClass();
        String f = fieldattributes.getName();
        boolean isSkip = false;

        if (owner == target) {
            if (ArrayUtils.contains(fields, f)) {
                log.debug("fitler field:{} for class:{}", f, owner);
                isSkip = true;
            }
            if (ArrayUtils.contains(clazz, c)) {
                log.debug("fitler class:{} for class:{}", c, owner);
                isSkip = true;
            }
            if (reverse) {
                isSkip = !isSkip;
            }
        }

        return isSkip;
    }

    public void setFields(String[] fields) {
        this.fields = fields;
    }

    public void setClazz(Class<?>[] clazz) {
        this.clazz = clazz;
    }

    public void setReverse(boolean reverse) {
        this.reverse = reverse;
    }
}

使用的时候是这样的

TargetStrategy ts = new TargetStrategy(Student.class);
//这里表示仅转换Student中的id和name属性
ts.setFields(new String[] {"id", "name"});
ts.setReverse(true);

Gson gson = new GsonBuilder().setExcludeStrategy(ts).create();
gson.toJson(teacher);

---

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HibernateProxy异常处理 
  在使用Hibernate时，那么很可能遇到这样的错误： 
java.lang.UnsupportedOperationException: Attempted to serialize java.lang.Class: org.hibernate.proxy.HibernateProxy. Forgot to register a type adapter? 
  因为gson在转换时是使用的反射机制，当获取的实体对象还在hibernate代理的时候，例如刚通过Id获取到，这时候获取到的便是代理对象HibernateProxy。这和直接调用实体对象的get方法不同，获取对象的属性就不能起作用。 
  解决的方法便是将代理对象实例化，见下面的代码

/**
 * This TypeAdapter unproxies Hibernate proxied objects, and serializes them
 * through the registered (or default) TypeAdapter of the base class.
 */
public class HibernateProxyTypeAdapter extends TypeAdapter<HibernateProxy> {

    public static final TypeAdapterFactory FACTORY = new TypeAdapterFactory() {
        @Override
        @SuppressWarnings("unchecked")
        public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
            return (HibernateProxy.class.isAssignableFrom(type.getRawType()) ? (TypeAdapter<T>) new HibernateProxyTypeAdapter(gson) : null);
        }
    };
    private final Gson context;

    private HibernateProxyTypeAdapter(Gson context) {
        this.context = context;
    }

    @Override
    public HibernateProxy read(JsonReader in) throws IOException {
        throw new UnsupportedOperationException("Not supported");
    }

    @SuppressWarnings({"rawtypes", "unchecked"})
    @Override
    public void write(JsonWriter out, HibernateProxy value) throws IOException {
        if (value == null) {
            out.nullValue();
            return;
        }
        // Retrieve the original (not proxy) class
        Class<?> baseType = Hibernate.getClass(value);
        // Get the TypeAdapter of the original class, to delegate the serialization
        TypeAdapter delegate = context.getAdapter(TypeToken.get(baseType));
        // Get a filled instance of the original class
        Object unproxiedValue = ((HibernateProxy) value).getHibernateLazyInitializer()
                .getImplementation();
        // Serialize the value
        delegate.write(out, unproxiedValue);
    }
}

使用的时候将该TypeAdapter的Factory注册到GsonBuilder,上面的代码变为

Gson gson = new GsonBuilder().setExcludeStrategy(ts)
.registerTypeAdapterFactory(HibernateProxyTypeAdapter.FACTORY)
.create();
gson.toJson(teacher);