Netty4源码分析之内存管理

在io.netty.buffer.AbstractByteBufAllocator类中有如下代码

//装饰器模式,用SimpleLeakAwareByteBuf或AdvancedLeakAwareByteBuf来包装原始的ByteBuf
//两个包装类均通过调用ResourceLeak的record方法来记录ByteBuf的方法调用堆栈,区别在于后者比前者记录更多的内容
protected static ByteBuf toLeakAwareBuffer(ByteBuf buf) {
    ResourceLeak leak;
    //根据设置的Level来选择使用何种包装器
    switch (ResourceLeakDetector.getLevel()) {
        case SIMPLE:
            //重建用于跟踪和表示内容泄露的ResourcLeak对象
            leak = AbstractByteBuf.leakDetector.open(buf);
            if (leak != null) {
                //只在ByteBuf.order方法中调用ResourceLeak.record
                buf = new SimpleLeakAwareByteBuf(buf, leak);
            }
            break;
        case ADVANCED:
        case PARANOID:
            leak = AbstractByteBuf.leakDetector.open(buf);
            if (leak != null) {
                //在ByteBuf几乎所有方法中调用ResourceLeak.record  
                buf = new AdvancedLeakAwareByteBuf(buf, leak);
            }
            break;
    }
    return buf;
}

内存泄露检测

//创建用于跟踪和表示内容泄露的ResourcLeak对象
public ResourceLeak open(T obj) {
    Level level = ResourceLeakDetector.level;
    if (level == Level.DISABLED) {//禁用内存跟踪
        return null;
    }
    if (level.ordinal() < Level.PARANOID.ordinal()) {
        //如果监控级别低于PARANOID,则只对要监控的资源进行采样
        if (leakCheckCnt ++ % samplingInterval == 0) {
            reportLeak(level);
            return new DefaultResourceLeak(obj);
        } else {
            return null;
        }
    } else {
        //否则对所有资源做监控
        reportLeak(level);
        return new DefaultResourceLeak(obj);
    }
}

其中reportLeak方法中完成对内存泄露的检测和报告,如下面代码所示.

private void reportLeak(Level level) {
    //......

    // 报告生成了太多的活跃资源
    int samplingInterval = level == Level.PARANOID? 1 : this.samplingInterval;
    if (active * samplingInterval > maxActive && loggedTooManyActive.compareAndSet(false, true)) {
        logger.error("LEAK: You are creating too many " + resourceType + " instances.  " +
                resourceType + " is a shared resource that must be reused across the JVM," +
                "so that only a few instances are created.");
    }

    // 检测并报告之前发生的内存泄露
    for (;;) {
        @SuppressWarnings("unchecked")
        //检查引用队列(为什么通过检查该队列,可以判断是否存在内存泄露)
        DefaultResourceLeak ref = (DefaultResourceLeak) refQueue.poll();
        if (ref == null) {//队列为空,没有未报告的内存泄露或者从未发生内存泄露
            break;
        }
	//清理引用
        ref.clear();

        if (!ref.close()) {
            continue;
        }
        //通过错误日志打印资源的方法调用记录,并将其保存在reportedLeaks中
        String records = ref.toString();
        if (reportedLeaks.putIfAbsent(records, Boolean.TRUE) == null) {
            if (records.isEmpty()) {
                logger.error("LEAK: {}.release() was not called before it‘s garbage-collected. " +
                        "Enable advanced leak reporting to find out where the leak occurred. " +
                        "To enable advanced leak reporting, " +
                        "specify the JVM option ‘-D{}={}‘ or call {}.setLevel()",
                        resourceType, PROP_LEVEL, Level.ADVANCED.name().toLowerCase(), simpleClassName(this));
            } else {
                logger.error(
                        "LEAK: {}.release() was not called before it‘s garbage-collected.{}",
                        resourceType, records);
            }
        }
    }
}

DefaultResourceLeak的声明如下

private final class DefaultResourceLeak extends PhantomReference<Object> implements ResourceLeak{
    //......

    public DefaultResourceLeak(Object referent) {
        //使用一个静态的引用队列(refQueue)初始化
        //refQueue是ResourceLeakDecetor的成员变量并由其初始化
        super(referent, referent != null? refQueue : null);
        //......
    }

    //......
}