• DirectByteBuffer中的address指向的地址空间属于堆外内存，它是不受JVM的管控，所以需要我们自己去管理；
• 前面的文章我们分析过address的地址由mmap或malloc的分配，那么其实DirectByteBuffer回收也是分为两处；

回收malloc分配的内存

当我们直接调用如下代码创建DirectByteBuffer，其内部的address由malloc分配(如果平台支持直接内存)

ByteBuffer.allocateDirect(1024)
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构造方法

DirectByteBuffer(int cap) {                   // package-privatesuper(-1, 0, cap, cap);boolean pa = VM.isDirectMemoryPageAligned();int ps = Bits.pageSize();long size = Math.max(1L, (long)cap + (pa ? ps : 0));Bits.reserveMemory(size, cap);long base = 0;try {base = unsafe.allocateMemory(size);} catch (OutOfMemoryError x) {Bits.unreserveMemory(size, cap);throw x;}unsafe.setMemory(base, size, (byte) 0);if (pa && (base % ps != 0)) {// Round up to page boundaryaddress = base + ps - (base & (ps - 1));} else {address = base;}cleaner = Cleaner.create(this, new Deallocator(base, size, cap));att = null;
}
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Cleaner其实是一个虚引用，Deallocator是一个Runnable

public class Cleaner extends PhantomReference<Object>
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回收的核心原理是虚引用，用于跟踪垃圾回收过程，可以参考我的“Java、Android引用类型”文章。

 private Cleaner(Object referent, Runnable thunk) {super(referent, dummyQueue);this.thunk = thunk;
}
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• referent：代表我们创建的DirectByteBuffer；
• dummyQueue：为Cleaner内部维护的引用队列；
  我们直接看父类Reference

static {ThreadGroup tg = Thread.currentThread().getThreadGroup();for (ThreadGroup tgn = tg;tgn != null;tg = tgn, tgn = tg.getParent());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();// provide access in SharedSecretsSharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() {@Overridepublic boolean tryHandlePendingReference() {return tryHandlePending(false);}});
}
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父类静态代码块创建了一个守护线程，当DirectByteBuffer对象从pending状态 ——> enqueue状态时，在handler的run方法中会检测到

 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' chainpending = 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 waitedreturn 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();// retryreturn true;} catch (InterruptedException x) {// retryreturn true;}// Fast path for cleanersif (c != null) {c.clean();return true;}ReferenceQueue<? super Object> q = r.queue;if (q != ReferenceQueue.NULL) q.enqueue(r);return true;}
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如果是c = r instanceof Cleaner ? (Cleaner) r : null;就会得到一个Cleaner，并且调用它的c.clean()方法，

public void clean() {if (!remove(this))return;try {thunk.run();} catch (final Throwable x) {AccessController.doPrivileged(new PrivilegedAction<Void>() {public Void run() {if (System.err != null)new Error("Cleaner terminated abnormally", x).printStackTrace();System.exit(1);return null;}});}}
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clean()方法最终回调用我们初始化时传进来的Deallocator

cleaner = Cleaner.create(this, new Deallocator(base, size, cap));
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在Deallocator中调用unsafe.freeMemory(address);进行回收

public void run() {if (address == 0) {// Paranoiareturn;}unsafe.freeMemory(address);address = 0;Bits.unreserveMemory(size, capacity);
}
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回收mmap分配的内存

我们看一次FileChannleImpl中map方法代码

int isize = (int)size;Unmapper um = new Unmapper(addr, mapSize, isize, mfd);if ((!writable) || (imode == MAP_RO)) {return Util.newMappedByteBufferR(isize,addr + pagePosition,mfd,um);} else {return Util.newMappedByteBuffer(isize,addr + pagePosition,mfd,um);}
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Util.newMappedByteBuffer调用用传入了一个Unmapper实例，Unmapper其实也是一个Runnable，到此其实大家也明白了，最终的回收还是在Unmapper中完成的

public void run() {if (address == 0)return;unmap0(address, size);address = 0;// if this mapping has a valid file descriptor then we close itif (fd.valid()) {try {nd.close(fd);} catch (IOException ignore) {// nothing we can do}}synchronized (Unmapper.class) {count--;totalSize -= size;totalCapacity -= cap;}
}
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最后调用底层的unmap0来完成内存的回收。