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 spaceat com.alibaba.fastjson.serializer.SerializeWriter.expandCapacity(SerializeWriter.java:at com.alibaba.fastjson.serializer.SerializeWriter.writeFieldNameDirect(SerializeWriterat 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:43at java.lang.reflect.Method.invoke(Method.java:498)at org.springframework.aop.aspectj.AbstractAspectJAdvice.invokeAdviceMethodWithGivenArgat org.springframework.aop.aspectj.AbstractAspectJAdvice.invokeAdviceMethod(AbstractAspat org.springframework.aop.aspectj.AspectJAroundAdvice.invoke(AspectJAroundAdvice.java:at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethoat org.springframework.aop.aspectj.MethodInvocationProceedingJoinPoint.proceed(MethodInat com.sinoiov.etc.aspect.SaasServiceAspect.aroundMethod(SaasServiceAspect.java:78)at sun.reflect.GeneratedMethodAccessor265.invoke(Unknown Source)at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43at java.lang.reflect.Method.invoke(Method.java:498)at org.springframework.aop.aspectj.AbstractAspectJAdvice.invokeAdviceMethodWithGivenArgat org.springframework.aop.aspectj.AbstractAspectJAdvice.invokeAdviceMethod(AbstractAspat org.springframework.aop.aspectj.AspectJAroundAdvice.invoke(AspectJAroundAdvice.java:at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethoat org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocatat org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethoat org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(at com.sinoiov.etc.finance.service.impl.TotalLoanServiceImpl$$EnhancerBySpringCGLIB$$94at com.sinoiov.etc.finance.api.impl.TotalLoanOperatorServiceImpl.queryTotalLoans(TotalLat 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$1.reply(DubboProtocol.java:108)at com.alibaba.dubbo.remoting.exchange.support.header.HeaderExchangeHandler.handleRequeat com.alibaba.dubbo.remoting.exchange.support.header.HeaderExchangeHandler.received(Heat com.alibaba.dubbo.remoting.transport.DecodeHandler.received(DecodeHandler.java:52)at com.alibaba.dubbo.remoting.transport.dispatcher.ChannelEventRunnable.run(ChannelEvenat java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)at java.util.concurrent.ThreadPoolExecutor$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 spaceat 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。