首页手记 java是如何做资源回收补救的

java是如何做资源回收补救的

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Java

学习java的过程，我们经常谈论一个对象的回收，尤其是资源类型，如果没有显示的关闭，对象就被回收了，说明出现了资源泄漏。java本身为了防止这种情况，做了一些担保的方式，确保可以让未关闭的资源合理回收掉。

finalize回收

finalize方式是java对象被回收时触发的一个方法。java的很多资源对象，都是在finalize中写了担保的方法。

    /**
     * Ensures that the <code>close</code> method of this file input stream is
     * called when there are no more references to it.
     *
     * @exception  IOException  if an I/O error occurs.
     * @see        java.io.FileInputStream#close()
     */
    protected void finalize() throws IOException {
        if ((fd != null) &&  (fd != FileDescriptor.in)) {
            /* if fd is shared, the references in FileDescriptor
             * will ensure that finalizer is only called when
             * safe to do so. All references using the fd have
             * become unreachable. We can call close()
             */
            close();
        }
    }

上面是FileInputStream的finalize方法，在方法被调用时，会检测文件描述符是否存在，如果存在的话就调用close方法。来确保资源的回收。

finalize方法在我们学习java的时候都并不推荐进行重写，也不推荐写复杂的逻辑在里面，主要是因为gc的时候，都会调用这个方法，如果执行的内容太多，就会导致gc被拖长。影响程序的正常运行。而且这里也只是做一个简单的担保。大部分希望的还是编写代码的人可以调用close。这样在做判断的时候就结束了，而不用真正的调用关闭的代码。

Cleaner回收

在DirectByteBuffer中，使用了一个Cleaner对象进行补救的。

    
      unsafe.setMemory(base, size, (byte) 0);
       if (pa && (base % ps != 0)) {
           // Round up to page boundary
           address = base + ps - (base & (ps - 1));
       } else {
           address = base;
       }
       cleaner = Cleaner.create(this, new Deallocator(base, size, cap));
       att = null;

申请完资源后，会创建一个Deallocator对象。

 private static class Deallocator
        implements Runnable
    {

        private static Unsafe unsafe = Unsafe.getUnsafe();

        private long address;
        private long size;
        private int capacity;

        private Deallocator(long address, long size, int capacity) {
            assert (address != 0);
            this.address = address;
            this.size = size;
            this.capacity = capacity;
        }

        public void run() {
            if (address == 0) {
                // Paranoia
                return;
            }
            unsafe.freeMemory(address);
            address = 0;
            Bits.unreserveMemory(size, capacity);
        }

    }

Deallocator的run方法中就进行了资源的释放。执行的时机就是靠 Cleaner来触发的。
Cleaner是PhantomReference的子类，PhantomReference是Reference的子类。
在中有一个ReferenceHandler

    private static class ReferenceHandler extends Thread {

他的run方法就是调用cleaner里的clean方法。这个线程是在静态块里启动起来的。

        Thread handler = new ReferenceHandler(tg, "Reference Handler");
        /* If there were a special system-only priority greater than
         * MAX_PRIORITY, it would be used here
         */
        handler.setPriority(Thread.MAX_PRIORITY);
        handler.setDaemon(true);
        handler.start();
        SharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() {
            @Override
            public boolean tryHandlePendingReference() {
                return tryHandlePending(false);
            }
        });

于此同时，并且给SharedSecrets设置了一个JavaLangRefAccess。
调用clean方法的过程在tryHandlePending里，这里的参数很重要。

 static boolean tryHandlePending(boolean waitForNotify) {
        Reference<Object> r;
        Cleaner c;
        try {
            synchronized (lock) {
                if (pending != null) {
                    r = pending;
                    // 'instanceof' might throw OutOfMemoryError sometimes
                    // so do this before un-linking 'r' from the 'pending' chain...
                    c = r instanceof Cleaner ? (Cleaner) r : null;
                    // unlink 'r' from 'pending' chain
                    pending = r.discovered;
                    r.discovered = null;
                } else {
                    // The waiting on the lock may cause an OutOfMemoryError
                    // because it may try to allocate exception objects.
                    if (waitForNotify) {
                        lock.wait();
                    }
                    // retry if waited
                    return waitForNotify;
                }
            }
        } catch (OutOfMemoryError x) {
            // Give other threads CPU time so they hopefully drop some live references
            // and GC reclaims some space.
            // Also prevent CPU intensive spinning in case 'r instanceof Cleaner' above
            // persistently throws OOME for some time...
            Thread.yield();
            // retry
            return true;
        } catch (InterruptedException x) {
            // retry
            return true;
        }

waitForNotify是true的时候，在没有回收对象的时候，会进入阻塞，然后等ooe。外层是个死循环，就会被再次调用到，下次进来的时候就可以出发clean了。
ReferenceHandler是管理机制的一种。
还有一种就是SharedSecrets调用tryHandlePending(false)。
在另外一个类，bits里

    final JavaLangRefAccess jlra = SharedSecrets.getJavaLangRefAccess();

        // retry while helping enqueue pending Reference objects
        // which includes executing pending Cleaner(s) which includes
        // Cleaner(s) that free direct buffer memory
        while (jlra.tryHandlePendingReference()) {
            if (tryReserveMemory(size, cap)) {
                return;
            }
        }