跟随狂神学Java-29，JUC并发编程

跟随狂神学Java

作者：joker2yue
链接：https://github.com/Joker2Yue/Joker2Yue-Blog
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第二十九天：JUC并发编程

“计算机是为了解决以前不存在的问题而诞生的。”

【狂神说Java】JUC并发编程最新版通俗易懂_哔哩哔哩_bilibili

线程和进程

---

进程可以理解为一个程序。程序的依次执行过程

一个进程往往可以包括多个线程

Java默认有几个线程？2个，main线程和gc线程

Java真的可以开线程吗？不行，只能通过本地方法去调用C++

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 */
            }
        }
    }
//本地方法，调用C++
    private native void start0();

并发、并行

---

并发：多线程操作同一个资源

• CPU一个核心，模拟出来多条线程。快速交替

并行：多个人一起行走

• CPU多个核心，多个核心可以同时执行

package com.joker_yue;

public class Test0 {
    public static void main(String[] args) {
        //获取CPU的核心数
        //CPU密集型，IO密集型
        System.out.println(Runtime.getRuntime().availableProcessors());

    }
}

并发编程的本质：充分利用CPU的性能

---

线程有几个状态：

6个，创建NEW，运行RUNNABLE，阻塞 BLOCKED,，无限等待WAITING，计时等待TIMED_WAITING与终结TERMINATED。

/**
 * A thread state.  A thread can be in one of the following states:
 * <ul>
 * <li>{@link #NEW}<br>
 *     A thread that has not yet started is in this state.
 *     </li>
 * <li>{@link #RUNNABLE}<br>
 *     A thread executing in the Java virtual machine is in this state.
 *     </li>
 * <li>{@link #BLOCKED}<br>
 *     A thread that is blocked waiting for a monitor lock
 *     is in this state.
 *     </li>
 * <li>{@link #WAITING}<br>
 *     A thread that is waiting indefinitely for another thread to
 *     perform a particular action is in this state.
 *     </li>
 * <li>{@link #TIMED_WAITING}<br>
 *     A thread that is waiting for another thread to perform an action
 *     for up to a specified waiting time is in this state.
 *     </li>
 * <li>{@link #TERMINATED}<br>
 *     A thread that has exited is in this state.
 *     </li>
 * </ul>
 *
 * <p>
 * A thread can be in only one state at a given point in time.
 * These states are virtual machine states which do not reflect
 * any operating system thread states.
 *
 * @since   1.5
 * @see #getState
 */
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 {@code Object.wait()}
     * on an object is waiting for another thread to call
     * {@code Object.notify()} or {@code Object.notifyAll()} on
     * that object. A thread that has called {@code Thread.join()}
     * 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;
}

---

WAIT和SLEEP的区别

1. 来自不同的类：wait：Object；sleep：Thread
2. 关于锁的释放：wait释放锁，sleep不会
3. 使用范围不同：wait只能在同步代码块中使用，sleep随便都可以使用
4. 是否捕获异常：wait不需要捕获异常，sleep必须捕获异常，因为可能超时等待
5. 是否需要唤醒：wait需要被唤醒，sleep不需要被唤醒

---

Lock锁

Synchronize

package com.joker_yue;

//基本的卖票例子

/*
 * 真正的多线程开发，公司中的开发
 * 线程  就是一个单独的资源类，没有任何附属的操作
 * */
public class Test01 {
    public static void main(String[] args) {
        //并发：多个线程操作同一个资源类，把资源丢入线程
        Ticket ticket = new Ticket();

        new Thread(() -> {
            for (int i = 0; i < 60; i++) {
                ticket.sale();
            }
        }, "A").start();

        new Thread(() -> {
            for (int i = 0; i < 60; i++) {
                ticket.sale();
            }
        }, "B").start();

        new Thread(() -> {
            for (int i = 0; i < 60; i++) {
                ticket.sale();
            }
        }, "C").start();
    }
}

//资源类
class  Ticket {
    //属性+方法
    private int number = 40;

    //卖票的方式
    public synchronized void sale() {
        if (number > 0) {
            System.out.println(Thread.currentThread().getName() + "卖出了第" + (number--) + "张票" + "，剩余" + number);
        }
    }
}

Lock锁

---

它们分别是可重用入（最常用的），读锁，写锁

使用下面的语句创建的锁默认为非公平锁

Lock lock = new ReentrantLock();

公平锁：很公平，得先来后到，不能插队

不公平锁：十分不公平，可以插队（默认）

package com.joker_yue;

//基本的卖票例子

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

/*
 * 真正的多线程开发，公司中的开发
 * 线程  就是一个单独的资源类，没有任何附属的操作
 * */
public class Test02 {
    public static void main(String[] args) {
        //并发：多个线程操作同一个资源类，把资源丢入线程
        Ticket ticket = new Ticket();

        //@FunctionInterface 函数式接口
        new Thread(() -> {for (int i = 0; i < 40; i++)ticket.sale();}, "A").start();
        new Thread(() -> {for (int i = 0; i < 40; i++)ticket.sale();}, "B").start();
        new Thread(() -> {for (int i = 0; i < 40; i++)ticket.sale();}, "C").start();

}}

//Lock三部曲
//1，new ReentrantLock();        创建锁
//2，lock.lock();                加锁
//3，finally=>lock.unlock();     解锁

class Ticket2 {
    //属性+方法
    private int number = 30;

    Lock lock = new ReentrantLock();

    //卖票的方式
    public synchronized void sale() {
        lock.lock();//加锁
        try {
            //业务代码
            if (number > 0) {
                System.out.println(Thread.currentThread().getName() + "卖出了第" + (number--) + "张票" + "，剩余" + number);
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();//解锁
        }
    }

}

---

Synchronized和Lock的关系

1. Synchronized是内置的关键字，而Lock是Java类

2. Synchronized无法判断锁的状态，而Lock可以判断是否获取到了锁

3. Synchronized会自动释放锁，Lock必须要手动释放锁，如果不释放锁，会死锁

4. Synchronized线程1（获得锁，阻塞）、线程2（等待）。Lock锁不一定会等待下去

   //尝试获取锁
   lock.tryLock();

5. Synchronized可重入锁，不可以中断的，非公平的；Lock可重入锁，可以判断是否中断，可以自己设置公平

6. Synchronized适合锁少量的代码同步问题，Lock适合锁大量的同步代码

---

锁是什么？ 如何判断锁谁?

锁是保证资源按照人们想要的方式方法进行共享

package com.joker_yue.pc;

/*线程之间的通信问题
 * 线程交替执行 A B 操作同一个变量 要使最终num=0
 * A num+1
 * B num-1
 * 通知 等待唤醒
 *  */

public class A {
    public static void main(String[] args) {
        Data data = new Data();
        new Thread(() -> {
            for (int i = 0; i < 10; i++) {
                try {
                    data.increment();
                } catch (InterruptedException e) {
                    throw new RuntimeException(e);
                }
            }
        }, "A").start();
        new Thread(() -> {
            for (int i = 0; i < 10; i++) {
                try {
                    data.decrement();
                } catch (InterruptedException e) {
                    throw new RuntimeException(e);
                }
            }
        }, "B").start();
    }
}


//判断等待，业务，通知
class Data {//数字类
    private int number = 0;

    //+1
    public synchronized void increment() throws InterruptedException {
        while (number != 0) {//while自旋锁
            //等待
            this.wait();
        }
        number++;
        System.out.println(Thread.currentThread().getName() + "=>" + number);
        //通知其他线程+1已经完成
        this.notifyAll();
    }

    public synchronized void decrement() throws InterruptedException {
        while (number == 0) {//while自旋锁
            //等待
            this.wait();
        }
        number--;
        System.out.println(Thread.currentThread().getName() + "=>" + number);
        //通知其他线程-1已经完成
        this.notifyAll();
    }
}

问题存在:如果有更多线程，运行期间可能会出现2，3等数字。是因为我们判断时只用了if判断，if判断只会判断一次，会产生虚假唤醒问题。所以需要使用while判断

虚假唤醒就是当一个条件满足时，很多线程都被唤醒了，但是只有其中部分是有用的唤醒，其它的唤醒都是无用功