大家好，我是烤鸭：

    今天又遇到OOM了,原因在于 fastjson 的 toJSONString。

    这是线上的报错信息。 将这个 TotalLoanDTO 对象 toJSONString 导致的OOM。日志如下。

2019-10-31 12:07:49.896 [TID:285.93.15724948639339801] [XNIO-1 task-14]  INFO  [class:com.sinoi logInfo==> 请求id:c8969c8a64c14a45b3079f218f7359a4,RPC调用服务异常,异常信息{}
com.alibaba.dubbo.rpc.service.GenericException: java.lang.OutOfMemoryError: Java heap space
java.lang.OutOfMemoryError: Java heap space
	at com.alibaba.fastjson.serializer.SerializeWriter.expandCapacity(SerializeWriter.java:
	at com.alibaba.fastjson.serializer.SerializeWriter.writeFieldNameDirect(SerializeWriter
	at com.alibaba.fastjson.serializer.ASMSerializer_22_TotalLoanDTO.write(Unknown Source)
	at com.alibaba.fastjson.serializer.ListSerializer.write(ListSerializer.java:126)
	at com.alibaba.fastjson.serializer.JSONSerializer.write(JSONSerializer.java:275)
	at com.alibaba.fastjson.JSON.toJSONString(JSON.java:648)
	at com.alibaba.fastjson.JSON.toJSONString(JSON.java:590)
	at com.alibaba.fastjson.JSON.toJSONString(JSON.java:555)
	at com.sinoiov.etc.aspect.ServiceLogAspect.getJson(ServiceLogAspect.java:98)
	at com.sinoiov.etc.aspect.ServiceLogAspect.loggerInf(ServiceLogAspect.java:81)
	at com.sinoiov.etc.aspect.ServiceLogAspect.aroundMethod(ServiceLogAspect.java:56)
	at sun.reflect.GeneratedMethodAccessor266.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.springframework.aop.aspectj.AbstractAspectJAdvice.invokeAdviceMethodWithGivenArg
	at org.springframework.aop.aspectj.AbstractAspectJAdvice.invokeAdviceMethod(AbstractAsp
	at org.springframework.aop.aspectj.AspectJAroundAdvice.invoke(AspectJAroundAdvice.java:
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMetho
	at org.springframework.aop.aspectj.MethodInvocationProceedingJoinPoint.proceed(MethodIn
	at com.sinoiov.etc.aspect.SaasServiceAspect.aroundMethod(SaasServiceAspect.java:78)
	at sun.reflect.GeneratedMethodAccessor265.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.springframework.aop.aspectj.AbstractAspectJAdvice.invokeAdviceMethodWithGivenArg
	at org.springframework.aop.aspectj.AbstractAspectJAdvice.invokeAdviceMethod(AbstractAsp
	at org.springframework.aop.aspectj.AspectJAroundAdvice.invoke(AspectJAroundAdvice.java:
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMetho
	at org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocat
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMetho
	at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(
	at com.sinoiov.etc.finance.service.impl.TotalLoanServiceImpl$$EnhancerBySpringCGLIB$$94
	at com.sinoiov.etc.finance.api.impl.TotalLoanOperatorServiceImpl.queryTotalLoans(TotalL

	at com.alibaba.dubbo.rpc.filter.GenericFilter.invoke(GenericFilter.java:91)
	at com.alibaba.dubbo.rpc.protocol.ProtocolFilterWrapper$1.invoke(ProtocolFilterWrapper.
	at com.alibaba.dubbo.rpc.filter.ClassLoaderFilter.invoke(ClassLoaderFilter.java:38)
	at com.alibaba.dubbo.rpc.protocol.ProtocolFilterWrapper$1.invoke(ProtocolFilterWrapper.
	at com.alibaba.dubbo.rpc.filter.EchoFilter.invoke(EchoFilter.java:38)
	at com.alibaba.dubbo.rpc.protocol.ProtocolFilterWrapper$1.invoke(ProtocolFilterWrapper.
	at com.alibaba.dubbo.rpc.protocol.dubbo.DubboProtocol$1.reply(DubboProtocol.java:108)
	at com.alibaba.dubbo.remoting.exchange.support.header.HeaderExchangeHandler.handleReque
	at com.alibaba.dubbo.remoting.exchange.support.header.HeaderExchangeHandler.received(He
	at com.alibaba.dubbo.remoting.transport.DecodeHandler.received(DecodeHandler.java:52)
	at com.alibaba.dubbo.remoting.transport.dispatcher.ChannelEventRunnable.run(ChannelEven
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:748)

本地写了个test类测试一下。
启动命令：设置最大内存 10M
VM options ： -Xmx10M

package src.rams.oom;

import com.alibaba.fastjson.JSON;

import java.util.ArrayList;
import java.util.List;

/**
 * @Author test
 * @Description //fastjson 测试 json的OOM
 * @Date 2019/11/4 10:19
 * @Param
 * @return
 */
public class JsonOOM {
    static class DumpTest{

    }
    public static void main(String[] args) throws Exception {
        List<DumpTest> list = new ArrayList<>();
        for (int i = 0; i < 54000; i++) {
            DumpTest dumpTest = new DumpTest();
            list.add(dumpTest);
        }
        String string = JSON.toJSONString(list);
        System.out.println(string);
    }
}

我们先看一下测试方法的报错信息。

Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
	at java.util.Arrays.copyOfRange(Arrays.java:3664)
	at java.lang.String.<init>(String.java:207)
	at com.alibaba.fastjson.serializer.SerializeWriter.toString(SerializeWriter.java:425)
	at com.alibaba.fastjson.JSON.toJSONString(JSON.java:650)
	at com.alibaba.fastjson.JSON.toJSONString(JSON.java:590)
	at com.alibaba.fastjson.JSON.toJSONString(JSON.java:555)
	at src.rams.oom.JsonOOM.main(JsonOOM.java:31)

跟随源码看一下。

SerializeWriter out = new SerializeWriter(null, defaultFeatures, features);
// 初始化char 数组
protected char buf[] = new char[2048];

后续的 toJSONString 。

public static char[] copyOfRange(char[] original, int from, int to) {
    int newLength = to - from;
    if (newLength < 0)
        throw new IllegalArgumentException(from + " > " + to);
    char[] copy = new char[newLength];
    System.arraycopy(original, from, copy, 0,
                     Math.min(original.length - from, newLength));
    return copy;
}

最后报在这一行：

char[] copy = new char[newLength];

由于数组较大，直接申请分配堆内存上的空间，导致堆内存溢出。

再看一下生产上的OOM事件。同样的地方，都是在申请新的字符数组的时候导致的OOM。

 public void expandCapacity(int minimumCapacity) {
    int newCapacity = (buf.length * 3) / 2 + 1;

    if (newCapacity < minimumCapacity) {
        newCapacity = minimumCapacity;
    }
    char newValue[] = new char[newCapacity];
    System.arraycopy(buf, 0, newValue, 0, count);
    buf = newValue;
}

总结：

    控制内存大小和json序列化对象的大小。如果有一个较大的list 去 toJsonString的话，占用的字符串长度不够用，会申请多余的字符数组，直到够用。如果有一个几万对象的list去toJsonString的话，那估计药丸。