[转帖]Cat导致内存不足原因分析

背景

线上几亿的数据在回刷的时候容器服务会出现OOM而重启，导致任务中断

内存泄露分析

jmap -histo pid 找出了有几十亿的java.lang.StackTraceElement对象，找不到被谁引用了
jmap -dump:format=b,file=heapdump.hprof pid dump内存
下载到本机mac上，用mat（MemoryAnalyzer）分析，得到内存泄露报告，看到内存全部被com.dianping.cat.message.internal.DefaultMessageManager$Context引用，找到了罪魁祸首

原因分析

com.dianping.cat.log4j.Log4j2Appender 在打印错误Exception的时候会调用Cat.logError方法

public class Log4j2Appender extends AbstractAppender {
  ....
    public void append(LogEvent event) {
        try {
            Level level = event.getLevel();
            if (level.isMoreSpecificThan(Level.WARN)) {
                this.logError(event);
            }
        } catch (Exception var3) {
            if (!this.ignoreExceptions()) {
                throw new AppenderLoggingException(var3);
            }
        }
    }
   ....
    private void logError(LogEvent event) {
        Throwable exception = event.getThrown();
        if (exception != null) {
            Message message = event.getMessage();
            if (message != null) {
                Cat.logError(message.getFormattedMessage(), exception);
            } else {
                Cat.logError(exception);
            }
        }
<span class="token punctuation">}</span>

}

Cat类的logError函数最终调用到了DefaultMessageManager.shouldLog方法

public class Cat {
    ...
    public static void logError(String message, Throwable cause) {
        try {
            getProducer().logError(message, cause);
        } catch (Exception var3) {
            errorHandler(var3);
        }
<span class="token punctuation">}</span>
<span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">logError</span><span class="token punctuation">(</span><span class="token class-name">Throwable</span> cause<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
    <span class="token keyword">try</span> <span class="token punctuation">{<!-- --></span>
        <span class="token function">getProducer</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">logError</span><span class="token punctuation">(</span>cause<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">Exception</span> var2<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
        <span class="token function">errorHandler</span><span class="token punctuation">(</span>var2<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

}

public class DefaultMessageProducer implements MessageProducer {
    public void logError(String message, Throwable cause) {
        if (Cat.getManager().isCatEnabled()) {
            if (this.shouldLog(cause)) {
               ....
            }
        } else {
            cause.printStackTrace();
        }
    }
        private boolean shouldLog(Throwable e) {
        return this.m_manager instanceof DefaultMessageManager ? ((DefaultMessageManager)this.m_manager).shouldLog(e) : true;
    }

DefaultMessageManager类的m_context在shouldLog的时候把异常堆栈保存下来了，如果Cat事务不关闭，随着异常越来越多就导致了内存溢出

public class DefaultMessageManager {
    private ThreadLocal<DefaultMessageManager.Context> m_context = new ThreadLocal();
    boolean shouldLog(Throwable e) {
        DefaultMessageManager.Context ctx = (DefaultMessageManager.Context)this.m_context.get();
        return ctx != null ? ctx.shouldLog(e) : true;
    }
<span class="token keyword">class</span> <span class="token class-name">Context</span> <span class="token punctuation">{<!-- --></span>
    <span class="token comment">// 内存不足就是由于错误堆栈信息没有限制导致的</span>
    <span class="token keyword">private</span> <span class="token class-name">Set</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Throwable</span><span class="token punctuation">&gt;</span></span> m_knownExceptions<span class="token punctuation">;</span>

    <span class="token keyword">public</span> <span class="token class-name">Context</span><span class="token punctuation">(</span><span class="token class-name">String</span> domain<span class="token punctuation">,</span> <span class="token class-name">String</span> hostName<span class="token punctuation">,</span> <span class="token class-name">String</span> ipAddress<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
        <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span>
        <span class="token keyword">this</span><span class="token punctuation">.</span>m_knownExceptions <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">HashSet</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">public</span> <span class="token keyword">boolean</span> <span class="token function">shouldLog</span><span class="token punctuation">(</span><span class="token class-name">Throwable</span> e<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>m_knownExceptions <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
            <span class="token keyword">this</span><span class="token punctuation">.</span>m_knownExceptions <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">HashSet</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>

        <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>m_knownExceptions<span class="token punctuation">.</span><span class="token function">contains</span><span class="token punctuation">(</span>e<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
            <span class="token keyword">return</span> <span class="token boolean">false</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{<!-- --></span>
            <span class="token comment">// 这里没有限制大小，只要有异常就往Set里面添加，这里应该做一个优化</span>
            <span class="token keyword">this</span><span class="token punctuation">.</span>m_knownExceptions<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span>e<span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token keyword">return</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>

}

}

解决方案

手动调用Cat.getManager().reset();方法清空保存的异常堆栈信息

    public void reset() {
        DefaultMessageManager.Context ctx = (DefaultMessageManager.Context)this.m_context.get();
        if (ctx != null) {
            if (ctx.m_totalDurationInMicros == 0L) {
                ctx.m_stack.clear();
                ctx.m_knownExceptions.clear();
                this.m_context.remove();
            } else {
                // 这里会释放错误日志堆栈信息
                ctx.m_knownExceptions.clear();
            }
        }
    }