Java并发编程实战(一)

作者：IT小白 | 2024-08-04 03:34:29

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java并发编程实战

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前言
一、一级标题1
总结

前言

晚上刷视频刷到河北王校长的bug排除视频，校长深厚的基本功令我汗颜，需要好好学习一下多线程的相关知识了，由此开了这个系列，以极客专栏的学习为主，记下笔记，辅以processon画流程图，尽可能提高自己的并发编程水平

参考链接：

一、一级标题1

　　Java 里 synchronized、wait()/notify() 相关的知识很琐碎，看懂难，会用更难。但实际上 synchronized、wait()、notify() 不过是操作系统领域里管程模型的一种实现而已，Java SDK 并发包里的条件变量 Condition 也是管程里的概念，synchronized、wait()/notify()、条件变量这些知识如果单独理解，自然是管中窥豹　　管程作为一种解决并发问题的模型，是继信号量模型之后的一项重大创新，它与信号量在逻辑上是等价的（可以用管程实现信号量，也可以用信号量实现管程），但是相比之下管程更易用。而且，很多编程语言都支持管程　　

其实并发编程可以总结为三个核心问题：分工、同步、互斥。：
　　
　　所谓分工指的是如何高效地拆解任务并分配给线程，而同步指的是线程之间如何协作，互斥则是保证同一时刻只允许一个线程访问共享资源。Java SDK 并发包很大部分内容都是按照这三个维度组织的，例如 Fork/Join 框架就是一种分工模式，CountDownLatch 就是一种典型的同步方式，而可重入锁则是一种互斥手段。
　　当把并发编程核心的问题搞清楚，再回过头来看 Java SDK 并发包，你会感觉豁然开朗，它不过是针对并发问题开发出来的工具而已，此时的 SDK 并发包可以任你“盘”了。
　　而且，这三个核心问题是跨语言的，你如果要学习其他语言的并发编程类库，完全可以顺着这三个问题按图索骥。Java SDK 并发包其余的一部分则是并发容器和原子类，这些比较容易理解，属于辅助工具，其他语言里基本都能找到对应的。

2.xxx：
　内容关键字，关键字onApplicationEvent，内容。

　　第三段关键字
第四段

在这里插入图片描述
原图中 wait 到 runnable 状态的转换中，join实际上是Thread类的方法，但这里写成了Object

jdk源码中的线程状态

public enum State {
        /**
         * Thread state for a thread which has not yet started.
         */
        NEW,

        /**
         * Thread state for a runnable thread.  A thread in the runnable
         * state is executing in the Java virtual machine but it may
         * be waiting for other resources from the operating system
         * such as processor.
         */
        RUNNABLE,

        /**
         * Thread state for a thread blocked waiting for a monitor lock.
         * A thread in the blocked state is waiting for a monitor lock
         * to enter a synchronized block/method or
         * reenter a synchronized block/method after calling
         * {@link Object#wait() Object.wait}.
         */
        BLOCKED,

        /**
         * Thread state for a waiting thread.
         * A thread is in the waiting state due to calling one of the
         * following methods:
         * <ul>
         *   <li>{@link Object#wait() Object.wait} with no timeout</li>
         *   <li>{@link #join() Thread.join} with no timeout</li>
         *   <li>{@link LockSupport#park() LockSupport.park}</li>
         * </ul>
         *
         * <p>A thread in the waiting state is waiting for another thread to
         * perform a particular action.
         *
         * For example, a thread that has called <tt>Object.wait()</tt>
         * on an object is waiting for another thread to call
         * <tt>Object.notify()</tt> or <tt>Object.notifyAll()</tt> on
         * that object. A thread that has called <tt>Thread.join()</tt>
         * is waiting for a specified thread to terminate.
         */
        WAITING,

        /**
         * Thread state for a waiting thread with a specified waiting time.
         * A thread is in the timed waiting state due to calling one of
         * the following methods with a specified positive waiting time:
         * <ul>
         *   <li>{@link #sleep Thread.sleep}</li>
         *   <li>{@link Object#wait(long) Object.wait} with timeout</li>
         *   <li>{@link #join(long) Thread.join} with timeout</li>
         *   <li>{@link LockSupport#parkNanos LockSupport.parkNanos}</li>
         *   <li>{@link LockSupport#parkUntil LockSupport.parkUntil}</li>
         * </ul>
         */
        TIMED_WAITING,

        /**
         * Thread state for a terminated thread.
         * The thread has completed execution.
         */
        TERMINATED;
    }
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Thread.init方法

 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();
        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) {
            	//获取security的ThreadGroup
                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();
            }
        }
        //尊重线程初始化穿入的threadgroup；次选System security mananger 的 tg；再次选 parent的 tg。

        /* 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);
            }
        }
		//new状态的线程会添加到ThreadGroup
        g.addUnstarted();

        this.group = g;
        //新线程的属性依赖于父线程
        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();
    }
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尊重线程初始化穿入的threadgroup；次选System security mananger 的 tg；再次选 parent的 tg。

private static synchronized long nextThreadID() {
        return ++threadSeqNumber;
    }
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synchronized
一个新构造的线程对象是由其parent线程来进行空间分配的，而child线程继承了parent是否为Daemon、优先级和加载资源的contextClassLoader以及可继承的ThreadLocal，同时还会分配一个唯一的（sync）ID来标识这个child线程。至此，一个能够运行的线程对象就初始化好了，在堆内存中等待着运行。

线程对象在初始化完成之后，调用start()方法就可以启动这个线程。线程start()方法的含义是：当前线程（即parent线程）同步告知Java虚拟机，只要线程规划器空闲，应立即启动调用start()方法的线程。

//避免多线程同时启动一个线程
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".
         */
         //private volatile int threadStatus = 0;
         //volitile进行二次保障，synchronized已经可以保障不会同时执行
        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 {
        	//native方法，告诉操作系统分配cpu时间片完成系统调度
        	//start0完全执行前，线程处于Ready状态
            start0();
            //完成后，只要cpu分配执行权，就处于Running状态
            //Running和Ready不是jdk定义的状态，统称为Runnable
            started = true;
        } finally {
            try {
                if (!started) {
                    group.threadStartFailed(this);
                }
            } catch (Throwable ignore) {
            	//start0如果出现异常，会直接反馈给调用线程，Main函数里面的thread.start方法。
            	//防止我们的thread.start方法感知不到异常导致程序的错误执行
                /* do nothing. If start0 threw a Throwable then
                  it will be passed up the call stack */
            }
        }
    }
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Thread.sleep()

 /**
     * 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.
     * 线程不会失去监控锁
     *
     * @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.
     * 			sleep方法响应中断
     */
//会释放cpu执行权
//native方法
public static native void sleep(long millis) throws InterruptedException;
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Object.wait()

 /**
     * Causes the current thread to wait until either another thread invokes the
     * {@link java.lang.Object#notify()} method or the
     * {@link java.lang.Object#notifyAll()} method for this object, or a
     * specified amount of time has elapsed.
     * <p>
     * The current thread must own this object's monitor.
     * <p>
     * This method causes the current thread (call it <var>T</var>) to
     * place itself in the wait set for this object and then to relinquish
     * any and all synchronization claims on this object. Thread <var>T</var>
     * becomes disabled for thread scheduling purposes and lies dormant
     * until one of four things happens:
     * <ul>
     * <li>Some other thread invokes the {@code notify} method for this
     * object and thread <var>T</var> happens to be arbitrarily chosen as
     * the thread to be awakened.
     * <li>Some other thread invokes the {@code notifyAll} method for this
     * object.
     * <li>Some other thread {@linkplain Thread#interrupt() interrupts}
     * thread <var>T</var>.
     * <li>The specified amount of real time has elapsed, more or less.  If
     * {@code timeout} is zero, however, then real time is not taken into
     * consideration and the thread simply waits until notified.
     * </ul>
     * The thread <var>T</var> is then removed from the wait set for this
     * object and re-enabled for thread scheduling. It then competes in the
     * usual manner with other threads for the right to synchronize on the
     * object; once it has gained control of the object, all its
     * synchronization claims on the object are restored to the status quo
     * ante - that is, to the situation as of the time that the {@code wait}
     * method was invoked. Thread <var>T</var> then returns from the
     * invocation of the {@code wait} method. Thus, on return from the
     * {@code wait} method, the synchronization state of the object and of
     * thread {@code T} is exactly as it was when the {@code wait} method
     * was invoked.
     * <p>
     * A thread can also wake up without being notified, interrupted, or
     * timing out, a so-called <i>spurious wakeup</i>.  While this will rarely
     * occur in practice, applications must guard against it by testing for
     * the condition that should have caused the thread to be awakened, and
     * continuing to wait if the condition is not satisfied.  In other words,
     * waits should always occur in loops, like this one:
     * <pre>
     *     synchronized (obj) {
     *         while (&lt;condition does not hold&gt;)
     *             obj.wait(timeout);
     *         ... // Perform action appropriate to condition
     *     }
     * </pre>
     * (For more information on this topic, see Section 3.2.3 in Doug Lea's
     * "Concurrent Programming in Java (Second Edition)" (Addison-Wesley,
     * 2000), or Item 50 in Joshua Bloch's "Effective Java Programming
     * Language Guide" (Addison-Wesley, 2001).
     *
     * <p>If the current thread is {@linkplain java.lang.Thread#interrupt()
     * interrupted} by any thread before or while it is waiting, then an
     * {@code InterruptedException} is thrown.  This exception is not
     * thrown until the lock status of this object has been restored as
     * described above.
     *
     * <p>
     * Note that the {@code wait} method, as it places the current thread
     * into the wait set for this object, unlocks only this object; any
     * other objects on which the current thread may be synchronized remain
     * locked while the thread waits.
     * <p>
     * This method should only be called by a thread that is the owner
     * of this object's monitor. See the {@code notify} method for a
     * description of the ways in which a thread can become the owner of
     * a monitor.
     *
     * @param      timeout   the maximum time to wait in milliseconds.
     * @throws  IllegalArgumentException      if the value of timeout is
     *               negative.
     * @throws  IllegalMonitorStateException  if the current thread is not
     *               the owner of the object's monitor.
     * @throws  InterruptedException if any thread interrupted the
     *             current thread before or while the current thread
     *             was waiting for a notification.  The <i>interrupted
     *             status</i> of the current thread is cleared when
     *             this exception is thrown.
     * @see        java.lang.Object#notify()
     * @see        java.lang.Object#notifyAll()
     */
    public final native void wait(long timeout) throws InterruptedException;
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Thread.join()
在很多情况下，主线程生成并起动了子线程，如果子线程里要进行大量的耗时的运算，主线程往往将于子线程之前结束，但是如果主线程处理完其他的事务后，需要用到子线程的处理结果，也就是主线程需要等待子线程执行完成之后再结束，这个时候就要用到join()方法了。

作用是阻塞当前线程的执行，等到被调用join的线程对象执行完毕才执行继续执行当前线程

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) {
        	//start开启后并且没有被终止
            while (isAlive()) {
            	//wait是Object的方法，这个是父类，这个wait方法前有一个隐藏的含义，this.wait()(不是很准确)
            	//当前线程类(Thread)有一个当前线程，当前线程(Thread类)这个对象释放了cpu，释放了锁
            	//Thread的join方法释放的是当前调用join方法的那个对象的锁
                wait(0);
            }
        } else {
            while (isAlive()) {
                long delay = millis - now;
                if (delay <= 0) {
                    break;
                }
                wait(delay);
                now = System.currentTimeMillis() - base;
            }
        }
    }
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具体要看当前的锁对象是谁，如果是调用join方法的锁对象，则释放，否则不释放

synchronized(obj){
    thread.join(); //join不释放锁
}
synchronized(thread){
    thread.join(); //join释放锁
}
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public class JoinRelase {
    static Object object = new Object();

    public static void main(String[] args) throws InterruptedException {

        for (int i = 0; i < 2; i++) {
            Thread thread = new Thread(new SubThread(), "Daemon Thread!" + i);
            thread.setName("thread-" + i);
            thread.start();
            Thread.sleep(100);
        }
    }

    static class SubThread implements Runnable {
        @SneakyThrows
        @Override
        public void run() {
            synchronized (Thread.currentThread()) {
                System.out.println("获取到锁！！！ThreadName: " + Thread.currentThread().getName());
                Thread.currentThread().join();
            }
        }
    }
}
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总结

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