一次异常处理,StackOverflowError

#####0.接手新项目最近接手一个项目,大部分功能都已经完成的半残项目.10+个工程相互调用,根本没有文档,就连注释都少的可怜。是不是有点蛋疼呢,或许每个换新项目的人都是这个感受吧。

#####1.接口没检查参数测试一个接口的时候，没有传入参数，居然返回我一堆java异常,StackOverflowError…

type Exception report
message org.apache.cxf.interceptor.Fault
description The server encountered an internal error (org.apache.cxf.interceptor.Fault) that prevented it from fulfilling this request.
exception
java.lang.RuntimeException: org.apache.cxf.interceptor.Fault
	...
root cause
org.apache.cxf.interceptor.Fault
	...
root cause
java.lang.StackOverflowError
	java.lang.System.arraycopy(Native Method)
	java.lang.String.getChars(String.java:854)
	java.lang.AbstractStringBuilder.append(AbstractStringBuilder.java:391)
	java.lang.StringBuffer.append(StringBuffer.java:224)
	java.io.StringWriter.write(StringWriter.java:84)
	com.google.gson.stream.JsonWriter.string(JsonWriter.java:534)
	com.google.gson.stream.JsonWriter.writeDeferredName(JsonWriter.java:402)
	com.google.gson.stream.JsonWriter.nullValue(JsonWriter.java:431)
	com.google.gson.stream.JsonWriter.value(JsonWriter.java:415)
	com.google.gson.internal.bind.TypeAdapters$13.write(TypeAdapters.java:362)
	com.google.gson.internal.bind.TypeAdapters$13.write(TypeAdapters.java:346)
	com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)
	com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:89)
	com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:195)
	com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)
	com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:89)
	com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:195)

#####2.捕获异常项目用cxf做的 WebService, 在这里catch到.

cxf-rt-core:AbstractInvoker.java

protected Object invoke(Exchange exchange, final Object serviceObject, Method m, List<Object> params) {
	Object res;
	try {
		Object[] paramArray = new Object[]{};
		if (params != null) {
			paramArray = params.toArray();
		}
		res = performInvocation(exchange, serviceObject, m, paramArray);
		if (exchange.isOneWay()) {
			return null;
		}
		return new MessageContentsList(res);
	} catch (InvocationTargetException e) {
		Throwable t = e.getCause();
		if (t == null) {
			t = e;
		}
		// ...
	}

#####3.异常点使用google的gson将数据转换为json格式.在 toJson 的时候抽象类TypeAdapter使用了TypeAdapterRuntimeTypeWrapper的实现

Gson.java

public void toJson(Object src, Type typeOfSrc, JsonWriter writer) throws JsonIOException {
TypeAdapter<?> adapter = getAdapter(TypeToken.get(typeOfSrc));
boolean oldLenient = writer.isLenient();
writer.setLenient(true);
boolean oldHtmlSafe = writer.isHtmlSafe();
writer.setHtmlSafe(htmlSafe);
boolean oldSerializeNulls = writer.getSerializeNulls();
writer.setSerializeNulls(serializeNulls);
try {
  ((TypeAdapter<Object>) adapter).write(writer, src);
} catch (IOException e) {
  throw new JsonIOException(e);
} finally {
  writer.setLenient(oldLenient);
  writer.setHtmlSafe(oldHtmlSafe);
  writer.setSerializeNulls(oldSerializeNulls);
}
}

在TypeAdapterRuntimeTypeWrapper中一直在循环调用自己类的write方法.

package com.google.gson.internal.bind;
final class TypeAdapterRuntimeTypeWrapper<T> extends TypeAdapter<T>

@SuppressWarnings({"rawtypes", "unchecked"})
@Override
public void write(JsonWriter out, T value) throws IOException {
// Order of preference for choosing type adapters
// First preference: a type adapter registered for the runtime type
// Second preference: a type adapter registered for the declared type
// Third preference: reflective type adapter for the runtime type (if it is a sub class of the declared type)
// Fourth preference: reflective type adapter for the declared type

TypeAdapter chosen = delegate;
Type runtimeType = getRuntimeTypeIfMoreSpecific(type, value);
if (runtimeType != type) {
  TypeAdapter runtimeTypeAdapter = context.getAdapter(TypeToken.get(runtimeType));
  if (!(runtimeTypeAdapter instanceof ReflectiveTypeAdapterFactory.Adapter)) {
	// The user registered a type adapter for the runtime type, so we will use that
	chosen = runtimeTypeAdapter;
  } else if (!(delegate instanceof ReflectiveTypeAdapterFactory.Adapter)) {
	// The user registered a type adapter for Base class, so we prefer it over the
	// reflective type adapter for the runtime type
	chosen = delegate;
  } else {
	// Use the type adapter for runtime type
	chosen = runtimeTypeAdapter;
  }
}
chosen.write(out, value);
}

#####4.测试不就是这么个对象么，为什么会使gson陷入循环呢？难道是gson的Bug?自己构建个对象来toJson试试.

RespObj obj = new RespObj();
obj.setCode("10199");
obj.setType("PARAMETEREXCEPTION");
obj.setMessage("java.lang.NullPointerException");
new Gson().toJson(obj);

自己构造了这个对象，调用gson并没有异常呢。

#####5.原因在哪？

对比了下，原来项目中，message并不是字符串”java.lang.NullPointerException”，而是 java.lang.NullPointerException 对象…

这么做就会看到给异常了：

new Gson().toJson(new NullPointerException());

#####6.关于异常处理结论

原来所有的异常继承自Exception，Exception继承 Throwable，Throwable有个this指针.导致toJson的时候产生了循环.好吧，Gson并没有这个bug，是开发人员自己作死
对用户输入进行检测
最好是不要直接把异常抛给用户，大部分用户是看不懂的
内部异常往外抛,在最外层捕获，以友好的方式显示给用户
在最外层统一处理,尽量避免在每个servlet去处理
而且配置404,500返回页面，不要直接将异常打印