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小編給大家分享一下java如何制作資源回收機(jī)制,希望大家閱讀完這篇文章后大所收獲,下面讓我們一起去探討方法吧!
學(xué)習(xí)java的過程,我們經(jīng)常談?wù)撘粋€(gè)對(duì)象的回收,尤其是資源類型,如果沒有顯示的關(guān)閉,對(duì)象就被回收了,說明出現(xiàn)了資源泄漏。java本身為了防止這種情況,做了一些擔(dān)保的方式,確??梢宰屛搓P(guān)閉的資源合理回收掉。
finalize回收
finalize方式是java對(duì)象被回收時(shí)觸發(fā)的一個(gè)方法。java的很多資源對(duì)象,都是在finalize中寫了擔(dān)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,在方法被調(diào)用時(shí),會(huì)檢測(cè)文件描述符是否存在,如果存在的話就調(diào)用close方法。來確保資源的回收。
finalize方法在我們學(xué)習(xí)java的時(shí)候都并不推薦進(jìn)行重寫,也不推薦寫復(fù)雜的邏輯在里面,主要是因?yàn)間c的時(shí)候,都會(huì)調(diào)用這個(gè)方法,如果執(zhí)行的內(nèi)容太多,就會(huì)導(dǎo)致gc被拖長(zhǎng)。影響程序的正常運(yùn)行。而且這里也只是做一個(gè)簡(jiǎn)單的擔(dān)保。大部分希望的還是編寫代碼的人可以調(diào)用close。這樣在做判斷的時(shí)候就結(jié)束了,而不用真正的調(diào)用關(guān)閉的代碼。
Cleaner回收
在DirectByteBuffer中,使用了一個(gè)Cleaner對(duì)象進(jìn)行補(bǔ)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;
申請(qǐng)完資源后,會(huì)創(chuàng)建一個(gè)Deallocator對(duì)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方法中就進(jìn)行了資源的釋放。執(zhí)行的時(shí)機(jī)就是靠 Cleaner來觸發(fā)的。
Cleaner是PhantomReference的子類,PhantomReference是Reference的子類。
在中有一個(gè)ReferenceHandler
private static class ReferenceHandler extends Thread {
他的run方法就是調(diào)用cleaner里的clean方法。這個(gè)線程是在靜態(tài)塊里啟動(dòng)起來的。
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); } });
于此同時(shí),并且給SharedSecrets設(shè)置了一個(gè)JavaLangRefAccess。
調(diào)用clean方法的過程在tryHandlePending里,這里的參數(shù)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的時(shí)候,在沒有回收對(duì)象的時(shí)候,會(huì)進(jìn)入阻塞,然后等ooe。外層是個(gè)死循環(huán),就會(huì)被再次調(diào)用到,下次進(jìn)來的時(shí)候就可以出發(fā)clean了。
ReferenceHandler是管理機(jī)制的一種。
還有一種就是SharedSecrets調(diào)用tryHandlePending(false)。
在另外一個(gè)類,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的時(shí)候,會(huì)從SharedSecrets來調(diào)用tryHandlePending(false)。這里又變相的進(jìn)行了一次回收。
java回收利用兩種機(jī)制。一種是finalize,一種是Cleaner。其中Cleaner一部分依賴oome觸發(fā)一次回收,一部分利用reserveMemory中做一次回收。
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