class FinalReference<T> extends Reference<T> {
    public FinalReference(T referent, ReferenceQueue<? super T> q) {
        super(referent, q);
    }
}

final class Finalizer extends FinalReference<Object> {
    //存放Finalizer的引用隊列
    private static ReferenceQueue<Object> queue = new ReferenceQueue<>();
    //當(dāng)前等待待執(zhí)行Object#finalize方法的Finalizer節(jié)點
    private static Finalizer unfinalized = null;
    //鎖對象
    private static final Object lock = new Object();
    //Finalizer鏈 后續(xù)節(jié)點與前驅(qū)節(jié)點
    private Finalizer next = null, prev = null;
    //私有構(gòu)造函數(shù)
    private Finalizer(Object finalizee) {
        super(finalizee, queue);
        //頭插法將當(dāng)前對象加入Finalizer鏈中
        add();
    }
    /* Invoked by VM */
    static void register(Object finalizee) {new Finalizer(finalizee);}
    //頭插法將當(dāng)前對象加入Finalizer鏈中
    private void add() {
        //獲取Finalizer類中全局鎖對象對應(yīng)moniter
        synchronized (lock) {
            if (unfinalized != null) {
                this.next = unfinalized;
                unfinalized.prev = this;
            }
            //更新等待待執(zhí)行Object#finalize方法的節(jié)點
            unfinalized = this;
        }
    }
}

private static class FinalizerThread extends Thread {
    private volatile boolean running;
    FinalizerThread(ThreadGroup g) {
        //這個便是一面提到dump線程時會出現(xiàn)的Finalizer線程的名字
        super(g, "Finalizer");
    }
    public void run() {
        // 避免重復(fù)調(diào)用run方法
        if (running)
            return;
        // Finalizer線程先于System.initializeSystemClass被調(diào)用。等待直到JavaLangAccess可以訪問
        while (!VM.isBooted()) {
            try {
                VM.awaitBooted();
            } catch (InterruptedException x) {
                // ignore and continue
            }
        }
        final JavaLangAccess jla = SharedSecrets.getJavaLangAccess();
        running = true;
        //守護線程一直運行
        for (;;) {
            try {
                //從ReferenceQueue中取出Finalizer
                Finalizer f = (Finalizer)queue.remove();
                //調(diào)用Finalizer引用對象重寫的finalize方法，內(nèi)部實現(xiàn)上會catch Throwable 異常，保證FinalizerThread線程一直能運行
                f.runFinalizer(jla);
            } catch (InterruptedException x) {
                // ignore and continue
            }
        }
    }
}
static {
    ThreadGroup tg = Thread.currentThread().getThreadGroup();
    for (ThreadGroup tgn = tg;
         tgn != null;
         tg = tgn, tgn = tg.getParent());     
    Thread finalizer = new FinalizerThread(tg);
    //線程優(yōu)先級沒有ReferenceHandler守護線程高
    finalizer.setPriority(Thread.MAX_PRIORITY - 2);
    //設(shè)置為守護線程
    finalizer.setDaemon(true);
    //啟動線程
    finalizer.start();
}

private void runFinalizer(JavaLangAccess jla) {
    synchronized (this) {
        //已從Finalizer鏈中摘除，則不再執(zhí)行Finalizer引用的對象的finalize方法
        if (hasBeenFinalized()) return;
        remove();
    }
    try {
        //獲取Finalizer引用的對象
        Object finalizee = this.get();
        if (finalizee != null && !(finalizee instanceof java.lang.Enum)) {
            /**JavaLangAccess實現(xiàn)內(nèi)部會調(diào)用Finalizer引用的對象的finalize方法
             * 實際是調(diào)用System#setJavaLangAccess方法實例化的JavaLangAccess對象
             */ 
            jla.invokeFinalize(finalizee);
            //清除棧中包含的該變量引用，以降低conservative GC 錯誤的保留該對象的機會
            finalizee = null;
        }
    } catch (Throwable x) { }
    super.clear();
}