java是怎么做资源回收补救的
本篇内容主要讲解“java是怎么做资源回收补救的”，感兴趣的朋友不妨来看看。本文介绍的方法操作简单快捷，实用性强。下面就让小编来带大家学习“java是怎么做资源回收补救的”吧!
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;
}
}
在做reserveMemory的时候，会从SharedSecrets来调用tryHandlePending(false)。这里又变相的进行了一次回收。
到此，相信大家对“java是怎么做资源回收补救的”有了更深的了解，不妨来实际操作一番吧！这里是亿速云网站，更多相关内容可以进入相关频道进行查询，关注我们，继续学习！