多线程<2>: java.lang.Thread

随笔记录，看到哪里记哪里

---

/**
 * Returns a reference to the currently executing thread object.
 *
 * @return  the currently executing thread.
 */
public static native Thread currentThread();

 博文1 是针对Thread.currentThread()的实践，有点不太好懂

 博文2 有详细的解释，但不是太深刻

20190614| 补充：Thread.currentThread() 获取的对象就是当前正在执行的线程，可以是main主线程，也可以是正在运行的新建的线程A，在run方法中Thread.currentThread() 等同于this

 

/**
 * A hint to the scheduler that the current thread is willing to yield
 * its current use of a processor. The scheduler is free to ignore this
 * hint.
 *
 * <p> Yield is a heuristic attempt to improve relative progression
 * between threads that would otherwise over-utilise a CPU. Its use
 * should be combined with detailed profiling and benchmarking to
 * ensure that it actually has the desired effect.
 *
 * <p> It is rarely appropriate to use this method. It may be useful
 * for debugging or testing purposes, where it may help to reproduce
 * bugs due to race conditions. It may also be useful when designing
 * concurrency control constructs such as the ones in the
 * {@link java.util.concurrent.locks} package.
 */
public static native void yield();

 博文1 

 

/**
 * Causes the currently executing thread to sleep (temporarily cease
 * execution) for the specified number of milliseconds, subject to
 * the precision and accuracy of system timers and schedulers. The thread
 * does not lose ownership of any monitors.
 *
 * @param  millis
 *         the length of time to sleep in milliseconds
 *
 * @throws  IllegalArgumentException
 *          if the value of {@code millis} is negative
 *
 * @throws  InterruptedException
 *          if any thread has interrupted the current thread. The
 *          <i>interrupted status</i> of the current thread is
 *          cleared when this exception is thrown.
 */
public static native void sleep(long millis) throws InterruptedException;

  博文1 一篇很有趣味的文章 

  博文2 讲的深刻清楚

 

/**
 * Causes this thread to begin execution; the Java Virtual Machine
 * calls the <code>run</code> method of this thread.
 * <p>
 * The result is that two threads are running concurrently: the
 * current thread (which returns from the call to the
 * <code>start</code> method) and the other thread (which executes its
 * <code>run</code> method).
 * <p>
 * It is never legal to start a thread more than once.
 * In particular, a thread may not be restarted once it has completed
 * execution.
 *
 * @exception  IllegalThreadStateException  if the thread was already
 *               started.
 * @see        #run()
 * @see        #stop()
 */
public synchronized void start() {
    /**
     * This method is not invoked for the main method thread or "system"
     * group threads created/set up by the VM. Any new functionality added
     * to this method in the future may have to also be added to the VM.
     *
     * A zero status value corresponds to state "NEW".
     */
    if (threadStatus != 0)
        throw new IllegalThreadStateException();

    /* Notify the group that this thread is about to be started
     * so that it can be added to the group's list of threads
     * and the group's unstarted count can be decremented. */
    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();

 博文1      博文2

源码不太难，自己看也一样可以理解

---

光看代码觉得差点什么，下面通过实践来探寻原理

// 跳过如下方法
Thread.start()
Thread.run()

• 构造方法

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);
}

看到如上构造器的第一感觉是什么？楼主的第一想法是为什么不用bean优化下，两个构造器能解决的事搞那么多让看那么多代码，但后来仔细想想这个想法是错误的，按楼主想的那样在基础上增加了阅读难度，增加了耦合度

• init方法

整个Thread中有两个init方法，而最终都是指向同一个init方法来处理初始化

/**
 * Initializes a Thread with the current AccessControlContext.
 * @see #init(ThreadGroup,Runnable,String,long,AccessControlContext,boolean)
 */
private void init(ThreadGroup g, Runnable target, String name,
                  long stackSize) {
    init(g, target, name, stackSize, null, true);
}

/**
 * Initializes a Thread.
 *
 * @param g the Thread group
 * @param target the object whose run() method gets called
 * @param name the name of the new Thread
 * @param stackSize the desired stack size for the new thread, or
 *        zero to indicate that this parameter is to be ignored.
 * @param acc the AccessControlContext to inherit, or
 *            AccessController.getContext() if null
 * @param inheritThreadLocals if {@code true}, inherit initial values for
 *            inheritable thread-locals from the constructing thread
 */
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(); // 从资源中获取一个线程
    SecurityManager security = System.getSecurityManager(); // 防止未知恶意java代码对象
    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();
        }
    }

    /* checkAccess regardless of whether or not threadgroup is
       explicitly passed in. */
    g.checkAccess();    // 检测线程的访问权限，有异常情况报错

    /*
     * 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();
    this.priority = parent.getPriority();
    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();
}

在线程构造方法中调用init方法始终 围绕 初始化所属线程组、名字、 Runnable、栈大小等信息；整个过程相当于配置了一个线程工厂，此时只是初始化了所有的配置，线程还没有真正创建，当然资源同样也还没有分配，只有在调用 start() 的时候线程才会真正创建；

 

• exit方法

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;
} 


void threadTerminated(Thread t) {
    synchronized (this) {
        remove(t);

        if (nthreads == 0) {
            notifyAll();
        }
        if (daemon && (nthreads == 0) &&
            (nUnstartedThreads == 0) && (ngroups == 0))
        {
            destroy();
        }
    }
}

private void remove(Thread t) {
    synchronized (this) {
        if (destroyed) {
            return;
        }
        for (int i = 0 ; i < nthreads ; i++) {
            if (threads[i] == t) {
                System.arraycopy(threads, i + 1, threads, i, --nthreads - i);
                // Zap dangling reference to the dead thread so that
                // the garbage collector will collect it.
                threads[nthreads] = null;
                break;
            }
        }
    }
}

exit 方法则是由系统调用，在 Thread 销毁前释放资源；

 

发现一篇针对Thread讲的很详细的博文，后面的内容暂时就不讲解了，等下次回顾探索一些细节时候再更新。