Java多线程基础之Thread类详解

一、线程

我们都知道，进程是系统运行程序的基本单位，打开电脑的任务管理器，就可以看到一个个运行中的进程。但是仔细观察可以发现每个进程之下，还有更多的线程。
线程是比进程更小、更轻量级的执行单位，每个进程都拥有自己的一块内存空间和变量资源等，然而同一个进程下的多个线程则共享数据和资源，所以不管线程的创建和销毁工作，还是在线程之间切换工作，都要比进程更加轻量级、消耗系统资源更少。

二、Java多线程编程

在没有接触多线程之前，我们编写的程序都只有一个作为程序入口的main函数，其实这正是一个线程Main，当然这是单线程下的编程。

1、创建多线程

初学Java多线程编程的时候，需要掌握两种创建多线程的方法：

• 声明一个Thread类的子类，子类中重写Thread类的run方法。
• 声明一个实现Runnable接口的类，类中实现run方法。

更推荐使用第二种方式创建多线程，Thread类本身也实现了Runnable接口。

public class ThreadTest {

    public static void main(String[] args) {

        MyThread t1 = new MyThread();
        MyThreadRunnable target = new MyThreadRunnable();
        Thread t2 = new Thread(target);

        //启动线程1
        t1.start();
        //启动线程2
        t2.start();

    }

}

//创建线程方式一
class MyThread extends Thread {

    @Override
    public void run() {
        for (int i = 0; i < 1000; i++) {
            System.out.println("方式一-->" + i);
        }
    }
}

//创建线程方式二
class MyThreadRunnable implements Runnable {

    @Override
    public void run() {
        for (int i = 0; i < 1000; i++) {
            System.out.println("方式二-->" + i);
        }
    }
}

2、线程状态

Java中线程可以有如下6中状态：

• NEW 新创建
• RUNNABLE 可运行
• BLOCKED 阻塞
• WAITING 等待
• TIMED WAITING 计时等待
• TERMINATED 终止

线程之间的状态转换如下图（图源于《Java并发编程艺术》）：

Java 线程状态变迁

三、Thread类详解

Thread类是java.lang包下的类，是多线程经常需要使用的类。因为笔者之前看过一些关于多线程的书和资料，包括《Java核心技术》的并发一章和其他文章等，但是始终无法理解其中原理，总是容易遗忘。索性这次结合着源码进行学习，笔者通过阅读Thread类的源码（笔者阅读的是JDK8的源码），并对常用的方法作出了自己的注释，确实学习效果要比之前只看资料来得扎实。
其中，笔者对一些初学者容易产生疑惑的地方都作出了注释，比如join方法的使用，线程中断等。
笔者建议大家可以将下面的代码复制进自己的一个单独的类文件，然后对比笔者的注释（这里可以通过<kbd>Ctrl</kbd>+<kbd>f</kbd>进行搜索，快速定位），结合自己阅读源码，去开始探索多线程领域。

/*
 * Copyright (c) 1994, 2016, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package java.lang;

import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.security.AccessController;
import java.security.AccessControlContext;
import java.security.PrivilegedAction;
import java.util.Map;
import java.util.HashMap;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.locks.LockSupport;
import sun.nio.ch.Interruptible;
import sun.reflect.CallerSensitive;
import sun.reflect.Reflection;
import sun.security.util.SecurityConstants;


/*
    在某个线程中创建一个新线程，新线程的优先级和创建新线程的线程一致，如果创建线程是守护线程的话，则新线程
    是守护线程。

    当Java虚拟机启动时，通常有一个非守护线程（即main线程，调用指定类的main方法），Java虚拟机会继续执行这些线程
    直到发生如下情形之一：
        · 调用了类Runtime的exit()方法，并且安全管理器允许发生退出操作。
        · 所有非守护线程均已死亡，可以是通过执行完run方法后返回，也可以是执行run方法时抛出了异常。

    创建新的执行线程的两种方法：
        · 声明一个Thread类的子类，子类中重写Thread类的run方法。
        · 声明一个实现Runnable接口的类，类中实现run方法。

    每个线程都有一个名字以供识别。线程名字可以相同。如果创建线程时未指定名字，会分配一个新名字。

    除非特殊说明，否则将null参数传递给Thread类的构造器或者方法中会导致NullPointerException。
 */
public
class Thread implements Runnable {
    /* Make sure registerNatives is the first thing <clinit> does. */
    private static native void registerNatives();
    static {
        registerNatives();
    }

    private volatile String name; //线程名称
    private int            priority; //线程优先级
    private Thread         threadQ;
    private long           eetop;

    /* Whether or not to single_step this thread. */
    private boolean     single_step;

    /* Whether or not the thread is a daemon thread. */
    private boolean     daemon = false; //是否守护线程标识，初始false

    /* JVM state */
    private boolean     stillborn = false;

    /* What will be run. */
    private Runnable target; //线程执行部分

    /* The group of this thread */
    private ThreadGroup group; //线程的线程组

    /* The context ClassLoader for this thread */
    private ClassLoader contextClassLoader; //线程的上下文类加载器

    /* The inherited AccessControlContext of this thread */
    private AccessControlContext inheritedAccessControlContext;

    /* For autonumbering anonymous threads. */
    private static int threadInitNumber; //静态域，用于分配线程名称
    private static synchronized int nextThreadNum() {
        return threadInitNumber++;
    }

    ThreadLocal.ThreadLocalMap threadLocals = null;

    ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;

    private long stackSize;

    /*
     * JVM-private state that persists after native thread termination.
     */
    private long nativeParkEventPointer;

    
    private long tid; //线程ID

    private static long threadSeqNumber; //静态域，用于分配线程ID

    private volatile int threadStatus = 0; //线程状态，初始值表示新创建还未启动

    //静态同步方法，用于返回分配给线程的ID
    private static synchronized long nextThreadID() {
        return ++threadSeqNumber;
    }

    volatile Object parkBlocker;

    private volatile Interruptible blocker;
    private final Object blockerLock = new Object(); //锁对象

    void blockedOn(Interruptible b) {
        synchronized (blockerLock) {
            blocker = b;
        }
    }
   
    public final static int MIN_PRIORITY = 1; //最小线程优先级

    public final static int NORM_PRIORITY = 5; //正常线程优先级

    public final static int MAX_PRIORITY = 10; //最大线程优先级

    //返回当前的执行线程对象
    public static native Thread currentThread();

    //静态方法，导致当前线程处于让步状态，表示当前线程愿意让出CPU资源使得其他线程执行
    public static native void yield();

    /*
        静态方法，使当前执行的线程休眠给定的毫秒数，该线程不会放弃所获取的所有锁
        如果某个线程正在sleep的过程中被别的线程打断，则该线程将抛出InterruptedException异常，并且该线程的中断位会被清除
     */
    public static native void sleep(long millis) throws InterruptedException;

    public static void sleep(long millis, int nanos)
    throws InterruptedException {
        if (millis < 0) {
            throw new IllegalArgumentException("timeout value is negative");
        }

        if (nanos < 0 || nanos > 999999) {
            throw new IllegalArgumentException(
                                "nanosecond timeout value out of range");
        }

        if (nanos >= 500000 || (nanos != 0 && millis == 0)) {
            millis++;
        }

        sleep(millis);
    }

    private void init(ThreadGroup g, Runnable target, String name,
                      long stackSize) {
        init(g, target, name, stackSize, null, true);
    }

    private void init(ThreadGroup g, Runnable target, String name,
                      long stackSize, AccessControlContext acc,
                      boolean inheritThreadLocals) {
        if (name == null) {
            throw new NullPointerException("name cannot be null");
        }

        this.name = name;

        Thread parent = currentThread(); //将创建新线程的线程作为父线程
        SecurityManager security = System.getSecurityManager();
        if (g == null) {
            /* Determine if it's an applet or not */

            /* If there is a security manager, ask the security manager
               what to do. */
            if (security != null) {
                g = security.getThreadGroup();
            }

            /* If the security doesn't have a strong opinion of the matter
               use the parent thread group. */
            if (g == null) {
                g = parent.getThreadGroup(); //和父线程同一个线程组
            }
        }

        /* checkAccess regardless of whether or not threadgroup is
           explicitly passed in. */
        g.checkAccess();

        /*
         * Do we have the required permissions?
         */
        if (security != null) {
            if (isCCLOverridden(getClass())) {
                security.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);
            }
        }

        g.addUnstarted();

        this.group = g;
        this.daemon = parent.isDaemon(); //新线程默认与父线程的守护属性保持一致
        this.priority = parent.getPriority(); //新线程默认与父线程的优先级保持一致
        if (security == null || isCCLOverridden(parent.getClass()))
            this.contextClassLoader = parent.getContextClassLoader();
        else
            this.contextClassLoader = parent.contextClassLoader;
        this.inheritedAccessControlContext =
                acc != null ? acc : AccessController.getContext();
        this.target = target;
        setPriority(priority);
        if (inheritThreadLocals && parent.inheritableThreadLocals != null)
            this.inheritableThreadLocals =
                ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
        /* Stash the specified stack size in case the VM cares */
        this.stackSize = stackSize;

        /* Set thread ID */
        tid = nextThreadID(); //设置线程ID
    }

    @Override
    protected Object clone() throws CloneNotSupportedException {
        throw new CloneNotSupportedException();
    }

    //可以发现：线程名称是采用 "Thread-" + 数字 的方式
    public Thread() {
        init(null, null, "Thread-" + nextThreadNum(), 0);
    }

    public Thread(Runnable target) {
        init(null, target, "Thread-" + nextThreadNum(), 0);
    }

    Thread(Runnable target, AccessControlContext acc) {
        init(null, target, "Thread-" + nextThreadNum(), 0, acc, false);
    }

    public Thread(ThreadGroup group, Runnable target) {
        init(group, target, "Thread-" + nextThreadNum(), 0);
    }

    public Thread(String name) {
        init(null, null, name, 0);
    }

    public Thread(ThreadGroup group, String name) {
        init(group, null, name, 0);
    }

    public Thread(Runnable target, String name) {
        init(null, target, name, 0);
    }

    public Thread(ThreadGroup group, Runnable target, String name) {
        init(group, target, name, 0);
    }

    public Thread(ThreadGroup group, Runnable target, String name,
                  long stackSize) {
        init(group, target, name, stackSize);
    }

    //同步方法
    public synchronized void start() {
        
        //start()方法只能调用一次，否则抛出该异常
        if (threadStatus != 0)
            throw new IllegalThreadStateException();

        group.add(this);

        boolean started = false;
        try {
            start0();
            started = true;
        } finally {
            try {
                if (!started) {
                    group.threadStartFailed(this);
                }
            } catch (Throwable ignore) {
                /* do nothing. If start0 threw a Throwable then
                  it will be passed up the call stack */
            }
        }
    }

    private native void start0();

    @Override
    public void run() {
        if (target != null) {
            target.run();
        }
    }

    private void exit() {
        if (group != null) {
            group.threadTerminated(this);
            group = null;
        }
        /* Aggressively null out all reference fields: see bug 4006245 */
        target = null;
        /* Speed the release of some of these resources */
        threadLocals = null;
        inheritableThreadLocals = null;
        inheritedAccessControlContext = null;
        blocker = null;
        uncaughtExceptionHandler = null;
    }

    @Deprecated
    public final void stop() {
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            checkAccess();
            if (this != Thread.currentThread()) {
                security.checkPermission(SecurityConstants.STOP_THREAD_PERMISSION);
            }
        }
        // A zero status value corresponds to "NEW", it can't change to
        // not-NEW because we hold the lock.
        if (threadStatus != 0) {
            resume(); // Wake up thread if it was suspended; no-op otherwise
        }

        // The VM can handle all thread states
        stop0(new ThreadDeath());
    }

    @Deprecated
    public final synchronized void stop(Throwable obj) {
        throw new UnsupportedOperationException();
    }

    /*
        中断此线程（即中断状态被置为true），除非该线程正在中断自己，否则总是允许的。

        如果该线程因为调用了Object类的wait()方法或者Thread类的join()、sleep()方法处于阻塞状态，则该线程
        的中断状态会被清除并且产生InterruptedException.
     */
    public void interrupt() {
        if (this != Thread.currentThread())
            checkAccess();

        synchronized (blockerLock) {
            Interruptible b = blocker;
            if (b != null) {
                interrupt0();           // Just to set the interrupt flag
                b.interrupt(this);
                return;
            }
        }
        interrupt0();
        interrupt0();
    }

    /*
        这是静态方法，检测当前线程是否处于中断状态，同时线程的中断状态会被该方法清除（即重置为false）。
     */
    public static boolean interrupted() {
        return currentThread().isInterrupted(true);
    }

    /*
        这是实例方法，检测该线程（调用该方法的线程实例）是否处于中断状态，调用该方法不会改变线程的中断状态。
     */
    public boolean isInterrupted() {
        return isInterrupted(false);
    }

    private native boolean isInterrupted(boolean ClearInterrupted);

    @Deprecated
    public void destroy() {
        throw new NoSuchMethodError();
    }

    //检测该线程是否处于活动状态，活动状态是指该线程被启动（调用start()方法）了并且没有死亡
    public final native boolean isAlive();

    @Deprecated
    public final void suspend() {
        checkAccess();
        suspend0();
    }

    @Deprecated
    public final void resume() {
        checkAccess();
        resume0();
    }

    //设置线程的优先级
    public final void setPriority(int newPriority) {
        ThreadGroup g;
        checkAccess();
        if (newPriority > MAX_PRIORITY || newPriority < MIN_PRIORITY) {//参数范围检验
            throw new IllegalArgumentException();
        }
        if((g = getThreadGroup()) != null) {
            if (newPriority > g.getMaxPriority()) {
                newPriority = g.getMaxPriority();
            }
            setPriority0(priority = newPriority);
        }
    }

    public final int getPriority() {
        return priority;
    }

    public final synchronized void setName(String name) {
        checkAccess();
        if (name == null) {
            throw new NullPointerException("name cannot be null");
        }

        this.name = name;
        if (threadStatus != 0) {
            setNativeName(name);
        }
    }

    public final String getName() {
        return name;
    }

    public final ThreadGroup getThreadGroup() {
        return group;
    }

    public static int activeCount() {
        return currentThread().getThreadGroup().activeCount();
    }

    public static int enumerate(Thread tarray[]) {
        return currentThread().getThreadGroup().enumerate(tarray);
    }

    @Deprecated
    public native int countStackFrames();

    public final synchronized void join(long millis)
    throws InterruptedException {
        long base = System.currentTimeMillis();
        long now = 0;

        if (millis < 0) {
            throw new IllegalArgumentException("timeout value is negative");
        }

        if (millis == 0) {
            while (isAlive()) {
                wait(0);
            }
        } else {
            while (isAlive()) {
                long delay = millis - now;
                if (delay <= 0) {
                    break;
                }
                wait(delay);
                now = System.currentTimeMillis() - base;
            }
        }
    }

    public final synchronized void join(long millis, int nanos)
    throws InterruptedException {

        if (millis < 0) {
            throw new IllegalArgumentException("timeout value is negative");
        }

        if (nanos < 0 || nanos > 999999) {
            throw new IllegalArgumentException(
                                "nanosecond timeout value out of range");
        }

        if (nanos >= 500000 || (nanos != 0 && millis == 0)) {
            millis++;
        }

        join(millis);
    }

    /*
        等待指定的线程终止。
        这里不少初学者都会有个疑惑：到底是谁等待谁终止？
        回答：假如在A线程中调用了B线程的join()方法（即B.join()），则表示A线程此时进入等待状态，
              会等待B线程终止才会继续执行。
     */
    public final void join() throws InterruptedException {
        join(0);
    }

    public static void dumpStack() {
        new Exception("Stack trace").printStackTrace();
    }

    /*
        设置此线程是否为守护线程，true表示守护线程，false表示用户进程。这个方法必须在调用该线程的start()
        方法之前调用，否则会抛出java.lang.IllegalThreadStateException

        当所有运行的线程都是守护线程时，Java虚拟机将会退出。
     */
    public final void setDaemon(boolean on) {
        checkAccess();
        if (isAlive()) {
            throw new IllegalThreadStateException();
        }
        daemon = on;
    }

    //检测该线程是否是守护线程
    public final boolean isDaemon() {
        return daemon;
    }

    public final void checkAccess() {
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkAccess(this);
        }
    }

    public String toString() {
        ThreadGroup group = getThreadGroup();
        if (group != null) {
            return "Thread[" + getName() + "," + getPriority() + "," +
                           group.getName() + "]";
        } else {
            return "Thread[" + getName() + "," + getPriority() + "," +
                            "" + "]";
        }
    }

    @CallerSensitive
    public ClassLoader getContextClassLoader() {
        if (contextClassLoader == null)
            return null;
        SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            ClassLoader.checkClassLoaderPermission(contextClassLoader,
                                                   Reflection.getCallerClass());
        }
        return contextClassLoader;
    }

    public void setContextClassLoader(ClassLoader cl) {
        SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            sm.checkPermission(new RuntimePermission("setContextClassLoader"));
        }
        contextClassLoader = cl;
    }

    public static native boolean holdsLock(Object obj);

    private static final StackTraceElement[] EMPTY_STACK_TRACE
        = new StackTraceElement[0];

    public StackTraceElement[] getStackTrace() {
        if (this != Thread.currentThread()) {
            // check for getStackTrace permission
            SecurityManager security = System.getSecurityManager();
            if (security != null) {
                security.checkPermission(
                    SecurityConstants.GET_STACK_TRACE_PERMISSION);
            }
            // optimization so we do not call into the vm for threads that
            // have not yet started or have terminated
            if (!isAlive()) {
                return EMPTY_STACK_TRACE;
            }
            StackTraceElement[][] stackTraceArray = dumpThreads(new Thread[] {this});
            StackTraceElement[] stackTrace = stackTraceArray[0];
            // a thread that was alive during the previous isAlive call may have
            // since terminated, therefore not having a stacktrace.
            if (stackTrace == null) {
                stackTrace = EMPTY_STACK_TRACE;
            }
            return stackTrace;
        } else {
            // Don't need JVM help for current thread
            return (new Exception()).getStackTrace();
        }
    }

    public static Map<Thread, StackTraceElement[]> getAllStackTraces() {
        // check for getStackTrace permission
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkPermission(
                SecurityConstants.GET_STACK_TRACE_PERMISSION);
            security.checkPermission(
                SecurityConstants.MODIFY_THREADGROUP_PERMISSION);
        }

        // Get a snapshot of the list of all threads
        Thread[] threads = getThreads();
        StackTraceElement[][] traces = dumpThreads(threads);
        Map<Thread, StackTraceElement[]> m = new HashMap<>(threads.length);
        for (int i = 0; i < threads.length; i++) {
            StackTraceElement[] stackTrace = traces[i];
            if (stackTrace != null) {
                m.put(threads[i], stackTrace);
            }
            // else terminated so we don't put it in the map
        }
        return m;
    }


    private static final RuntimePermission SUBCLASS_IMPLEMENTATION_PERMISSION =
                    new RuntimePermission("enableContextClassLoaderOverride");

    private static class Caches {
        /** cache of subclass security audit results */
        static final ConcurrentMap<WeakClassKey,Boolean> subclassAudits =
            new ConcurrentHashMap<>();

        /** queue for WeakReferences to audited subclasses */
        static final ReferenceQueue<Class<?>> subclassAuditsQueue =
            new ReferenceQueue<>();
    }

    private static boolean isCCLOverridden(Class<?> cl) {
        if (cl == Thread.class)
            return false;

        processQueue(Caches.subclassAuditsQueue, Caches.subclassAudits);
        WeakClassKey key = new WeakClassKey(cl, Caches.subclassAuditsQueue);
        Boolean result = Caches.subclassAudits.get(key);
        if (result == null) {
            result = Boolean.valueOf(auditSubclass(cl));
            Caches.subclassAudits.putIfAbsent(key, result);
        }

        return result.booleanValue();
    }

    private static boolean auditSubclass(final Class<?> subcl) {
        Boolean result = AccessController.doPrivileged(
            new PrivilegedAction<Boolean>() {
                public Boolean run() {
                    for (Class<?> cl = subcl;
                         cl != Thread.class;
                         cl = cl.getSuperclass())
                    {
                        try {
                            cl.getDeclaredMethod("getContextClassLoader", new Class<?>[0]);
                            return Boolean.TRUE;
                        } catch (NoSuchMethodException ex) {
                        }
                        try {
                            Class<?>[] params = {ClassLoader.class};
                            cl.getDeclaredMethod("setContextClassLoader", params);
                            return Boolean.TRUE;
                        } catch (NoSuchMethodException ex) {
                        }
                    }
                    return Boolean.FALSE;
                }
            }
        );
        return result.booleanValue();
    }

    private native static StackTraceElement[][] dumpThreads(Thread[] threads);
    private native static Thread[] getThreads();

    public long getId() {
        return tid;
    }

    /*
        线程的六种状态：
            NEW           新创建
            RUNNABLE      可运行
            BLOCKED       阻塞
            WAITING       等待
            TIMED_WAITING 计时等待
            TERMINATED    终止
     */
    public enum State {
        //实例化一个线程后还没有调用其start()方法，处于新创建状态
        NEW,

        //处于可运行状态的线程可能正在运行也可能未运行（等待操作系统调度）
        RUNNABLE,

        //尝试获取锁但是未成功，等待锁
        BLOCKED,

        /*
            一个线程调用了下面三个方法会进入等待状态：
                Object.wait()
                Thread.join()
                LockSupport.park()
         */
        WAITING,

        /*
            一个线程调用了下面的方法会进入超时等待状态：
                Thread.sleep()
                带参数的Object.wait()
                带参数的Thread.join()
                LockSupport.parkNanos()
                LockSupport.parkUntil()
         */
        TIMED_WAITING,

        //run方法执行结束返回后或者run方法抛出未捕获的异常会导致线程终止
        TERMINATED;
    }

    /*
        返回该线程的状态
     */
    public State getState() {
        // get current thread state
        return sun.misc.VM.toThreadState(threadStatus);
    }

    // Added in JSR-166

    @FunctionalInterface
    public interface UncaughtExceptionHandler {
        void uncaughtException(Thread t, Throwable e);
    }

    // null unless explicitly set
    private volatile UncaughtExceptionHandler uncaughtExceptionHandler;

    // null unless explicitly set
    private static volatile UncaughtExceptionHandler defaultUncaughtExceptionHandler;

    public static void setDefaultUncaughtExceptionHandler(UncaughtExceptionHandler eh) {
        SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            sm.checkPermission(
                new RuntimePermission("setDefaultUncaughtExceptionHandler")
                    );
        }

         defaultUncaughtExceptionHandler = eh;
     }

    public static UncaughtExceptionHandler getDefaultUncaughtExceptionHandler(){
        return defaultUncaughtExceptionHandler;
    }

    public UncaughtExceptionHandler getUncaughtExceptionHandler() {
        return uncaughtExceptionHandler != null ?
            uncaughtExceptionHandler : group;
    }

    public void setUncaughtExceptionHandler(UncaughtExceptionHandler eh) {
        checkAccess();
        uncaughtExceptionHandler = eh;
    }

    private void dispatchUncaughtException(Throwable e) {
        getUncaughtExceptionHandler().uncaughtException(this, e);
    }

    static void processQueue(ReferenceQueue<Class<?>> queue,
                             ConcurrentMap<? extends
                             WeakReference<Class<?>>, ?> map)
    {
        Reference<? extends Class<?>> ref;
        while((ref = queue.poll()) != null) {
            map.remove(ref);
        }
    }

    static class WeakClassKey extends WeakReference<Class<?>> {
        private final int hash;

        WeakClassKey(Class<?> cl, ReferenceQueue<Class<?>> refQueue) {
            super(cl, refQueue);
            hash = System.identityHashCode(cl);
        }

        @Override
        public int hashCode() {
            return hash;
        }

        @Override
        public boolean equals(Object obj) {
            if (obj == this)
                return true;

            if (obj instanceof WeakClassKey) {
                Object referent = get();
                return (referent != null) &&
                       (referent == ((WeakClassKey) obj).get());
            } else {
                return false;
            }
        }
    }

    @sun.misc.Contended("tlr")
    long threadLocalRandomSeed;

    @sun.misc.Contended("tlr")
    int threadLocalRandomProbe;

    @sun.misc.Contended("tlr")
    int threadLocalRandomSecondarySeed;

    /* Some private helper methods */
    private native void setPriority0(int newPriority);
    private native void stop0(Object o);
    private native void suspend0();
    private native void resume0();
    private native void interrupt0();
    private native void setNativeName(String name);
}