Summary of Java knowledge points: multi-threading basics

This article brings you relevant knowledge about java, which mainly introduces the relevant content about multi-threading. Multiple threads can be executed at the same time. For example: You can use Thunder to download multiple files at the same time. Let’s take a look at it. I hope it will be helpful to everyone.

Recommended study: "java video tutorial"

1. Thread related concepts

1, Program: It is a set of instructions written in a certain language to complete a specific task. To put it simply: is the code we wrote.

2. Process: refers to a running program. For example, when we use QQ, we start a process, and the operating system will allocate memory space for the process. When we use Thunder and start another process, the operating system will allocate new memory space for Thunder.

A process is an execution of a program, or a program that is running. It is a dynamic process: it has its own process of emergence, existence and demise.

3, Thread: It is created by the process and is the entity of the process. A process can have multiple threads. For example, using Thunder to download files, Thunder is equivalent to the process , the downloaded file is equivalent to a thread.

4, Single thread: At the same time, is only allowed to execute one thread.

5, Multiple threads: At the same time, can execute multiple threads. For example: using Thunder you can download multiple files at the same time.

6, Concurrency: At the same time, multiple tasks are performed alternately. Multitasking achieved by a single-core CPU is concurrency.

7, Parallel: At the same time, multiple tasks are performed simultaneously. Multi-core CPUs can achieve parallelism. When there are many tasks, concurrency and parallelism may occur at the same time.

2. Basic use of threads

There are two ways to create a thread:

1. Inherit the Thread class and override it runMethod.

2. Implement the Runnable interface and override the run method.

Note: The Thread class implements the Runnable interface.

(1) Inherit the Thread class and override the run method

public class Thread01 {
    public static void main(String[] args) throws InterruptedException {
        Cat cat = new Cat();
        cat.start();
        System.out.println("主线程" + Thread.currentThread().getName());
        for (int i = 1; i <p>1. Inherit the <code>Thread</code> class and override the <code>run method ()</code>After the method, you need to create an object in the <code>main</code> method and call the <code>start()</code> method to start the thread. </p><p>2. Using the <code>start()</code> method will call the rewritten <code>run()</code> method. </p><p>3. If in the <code>main</code> method, there are execution statements after the <code>start()</code> method, and there are also execution statements in the <code>run()</code> method , <code>main</code> A sub-thread <code>Thread-0</code> will be started in the thread. The main thread will not block, and the main thread and sub-threads will execute alternately. </p><p>Note: If the main thread completes execution, but the child thread has not completed execution, the process will not end. After all threads have completed execution, the process will automatically end. </p><p>4. Why use the <code>start()</code> method to call the <code>run()</code> method in the main thread instead of directly calling the <code>run()</code> method, because the <code>run()</code> method is an ordinary method and does not actually start the thread. If the <code>run()</code> method is called, the <code>run()</code> method will be executed. After completion, if the remaining statements of the <code>main</code> method are executed, the main thread will be blocked. </p><p>So the result of the above program is: </p><pre class="brush:php;toolbar:false">主线程main
主线程i=1你好1Thread-0主线程i=2你好2Thread-0主线程i=3你好3Thread-0主线程i=4你好4Thread-0主线程i=5你好5Thread-0

(2) Implement the Runnable interface and override the run method

public class Thread02 {
    public static void main(String[] args) {
        Dog dog = new Dog();
        Thread thread = new Thread(dog);
        thread.start();
    }}class Dog implements Runnable{
    @Override    public void run() {
        int count = 0;
        while (true) {
            System.out.println("小狗汪汪叫" + (++count) + Thread.currentThread().getName());
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            if (count == 10){
                break;
            }
        }
    }}

1. Because## There is no start() method in the #Runnable interface, so agent is required.

2. First, create an object in the main method, then create a

Thread object, pass the previously created object into Thread, and use Thread to call start()Method to create a thread.

3. It is recommended to use

Runnable, because implementing the Runnable interface is more suitable for multiple threads to share a resource and avoids the limitations of single inheritance.

3. Thread termination

(1) Basic instructions

1. When the thread completes the task, it will automatically exit .

2. You can also use variables to control the

run() method to exit the termination thread.

(2) Operation

1. If the

run() method in a thread is internally a while(true){ }, that is, an infinite loop.

2. We can create a

boolean attribute loop inside the thread, let loop = true, and then while(loop){}.

3. Provide another

setLoop method, so that you can call the setLoop method in other classes and change the boolean value to control thread termination.

(3) Code demonstration

public class ThreadExit {
    public static void main(String[] args) throws InterruptedException {
        T t = new T();
        t.start();
        Thread.sleep(10000);
        t.setLoop(false);
    }}class T extends Thread{
    private boolean loop = true;
    private int times = 0;
    
    @Override    public void run() {
        while (loop){
            System.out.println("hello" + (++times));
            try {
                Thread.sleep(100);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public void setLoop(boolean loop) {
        this.loop = loop;
    }}

4. Common thread methods

(1) The first group

1,

setName: Set the thread name to be the same as the parameter name.

2、getName：返回该线程名称。

3、start：使该线程开始执行，JAVA虚拟机底层调用该线程的start0方法。

4、run：调用线程对象的run方法。

5、setPriority：更改线程的优先级。

6、getPriority：获取线程的优先级。

7、sleep：在指定的毫秒数内，让当前正在执行的线程休眠。

8、interrupt：中断线程。

注意事项：

1、start()底层会创建新的线程，调用run()，run()就是一个简单的方法调用，不会启动新线程。

2、中断线程一般用于中断正在休眠的线程，并没有真正的结束线程，所以如果线程中每输出内容后，会调用Thread.sleep()进行休眠的话，我们可以调用interrupt()方法将其提前唤醒。

代码演示：

public class ThreadMethod01 {
    public static void main(String[] args) throws InterruptedException {
        T t = new T();
        t.setName("邱崇源");
        t.setPriority(1);
        t.start();
        for (int i = 0; i <h4>
<a id="_183"></a>(二)第二组</h4><p>1、<strong>yield</strong>：是一个<code>静态方法</code>，表示<code>线程的礼让</code>。让出<code>CPU</code>，让其他线程执行，但是<code>不一定成功</code>，因为这取决于CPU，如果CPU认为两个线程可以一起执行，则不进行礼让，所以<code>如果CPU的核数多，并且线程数少，礼让就会大概率失败</code>。</p><p>2、<strong>join</strong>：表示<code>线程的插队</code>，假如主线程与子线程正在交替运行，我们想<code>先让子线程执行</code>完毕，然后再让主线程执行，就可以使用线程插队，在主线程中，创建子线程对象，并调用<code>join</code>方法，可以实现线程插队，线程插队<code>一定会成功</code>，先<code>执行完插入线程任务后，再继续执行主线程</code>。</p><h4>
<a id="_186"></a>代码演示：</h4><pre class="brush:php;toolbar:false">public class ThreadMethod02 {
    public static void main(String[] args) throws InterruptedException {
        A a = new A();
        a.start();
        for (int i = 1; i <h2>
<a id="_219"></a>五、用户线程和守护线程</h2><p>1、<strong>用户线程</strong>：也叫<code>工作线程</code>，线程的任务执行完或通知方式结束。</p><p>2、<strong>守护线程</strong>：一般是<code>为工作线程服务</code>的，当<code>所有的用户线程结束</code>，<code>守护线程自动结束</code>。</p><h4>
<a id="_222"></a>应用场景：</h4><p>如果有两个线程，主线程运行结束，但子线程是无限循环。我们想让主线程结束的同时，子线程也结束，就需要让子线程变成守护线程。</p><p>在主线程中，创建子线程对象，并调用<code>setDaemon（true）</code>，让子线程变成守护线程。</p><p>注意：一定要放在<code>start方法之前</code>。</p><h4>
<a id="_226"></a>代码演示：</h4><pre class="brush:php;toolbar:false">public class ThreadMethod03 {
    public static void main(String[] args) throws InterruptedException {
        MyDaemonThread myDaemonThread = new MyDaemonThread();
        myDaemonThread.setDaemon(true);
        myDaemonThread.start();
        for (int i = 1; i <h2>
<a id="_254"></a>六、线程的生命周期</h2><h4>
<a id="1JDK__ThreadState__255"></a>1、JDK 中用 Thread.State 枚举表示了线程的几种状态</h4><p><img src="/static/imghw/default1.png" data-src="https://img.php.cn/upload/article/000/000/067/2235493b090cc60a6a3c1434c8852f59-0.png" class="lazy" alt="Summary of Java knowledge points: multi-threading basics"></p><h4>
<a id="2_257"></a>2、线程状态转换图</h4><p><img src="/static/imghw/default1.png" data-src="https://img.php.cn/upload/article/000/000/067/cfbba58b413c41b084c9e0cab6295dfe-1.png" class="lazy" alt="Summary of Java knowledge points: multi-threading basics"></p><h2>
<a id="_259"></a>七、线程的同步</h2><h4>
<a id="1_260"></a>1、应用场景：</h4><p>假如有100张票，有三个线程同时进入该方法买票，票就有可能超卖。所以我们需要线程同步机制，保证数据在同一时刻，最多有一个线程访问。</p><p>可以采取同步方法，在方法中加入<code>synchronized</code>关键字。</p><p>也可以采用同步代码块，<code>synchronized（对象）{}</code>。</p><p>注意：<code>synchronized是非公平锁</code>，如果这次第一个线程访问了数据，那么下一次第一个线程也有可能访问到数据。</p><p>如果同步方法是<code>非静态</code>的，那么锁可以是this，也可以是其他对象，但要求是同一个对象。</p><p>例：<code>synchronized（this）</code>。</p><p>如果同步方法是<code>静态</code>的，锁为当前类本身。</p><p>例：<code>synchronized（类名：class）</code>。</p><h4>
<a id="2_270"></a>2、代码演示：</h4><pre class="brush:php;toolbar:false">public class SellTicket {
    public static void main(String[] args) {
        SellTicket02 sellTicket04 = new SellTicket02();
        Thread thread1 = new Thread(sellTicket04);
        Thread thread2 = new Thread(sellTicket04);
        Thread thread3 = new Thread(sellTicket04);
        thread1.start();
        thread2.start();
        thread3.start();
    }}class SellTicket02 implements Runnable {
    public static int ticket = 100;
    private boolean loop = true;

    public synchronized void sell() {
        if (ticket <h2>
<a id="_309"></a>八、线程的死锁</h2><h4>
<a id="1_310"></a>1、基本介绍</h4><p>多个线程都占用了对方的锁资源，但不肯相让，就导致了死锁，在编程中，一定要避免死锁的发生。</p><h4>
<a id="2_312"></a>2、发生场景：</h4><p>例如：A和B的面前都各有两道门，A的第一道门是o1，第二道门是o2。B的第一道门是o2，第二道门是o1。他们的面前有两把锁，一个是o1锁，一个是o2锁，假如A抢到了o1锁，B抢到了o2锁，但是他们只可打开各自的第一道门，第二道门都没有打开，那么他们都无法释放自己的锁资源，又不可能相让，因此发生了死锁。</p><h4>
<a id="3_314"></a>3、代码演示：</h4><pre class="brush:php;toolbar:false">public class DeadLock_ {
	public static void main(String[] args) { //模拟死锁现象 
		DeadLockDemo A = new DeadLockDemo(true); 
		A.setName("A 线程"); 
		DeadLockDemo B = new DeadLockDemo(false); 
		B.setName("B 线程");
		A.start(); 
		B.start(); 
	} }class DeadLockDemo extends Thread { 
	static Object o1 = new Object();// 保证多线程，共享一个对象,这里使用 static 
	static Object o2 = new Object(); 
	boolean flag; 
	
	public DeadLockDemo(boolean flag) {//构造器 
		this.flag = flag; 
	}
	
	@Override 
	public void run() {  
		if (flag) { 
			synchronized (o1) { 
				System.out.println(Thread.currentThread().getName() + " 进入 1"); 
				synchronized (o2) { 
					System.out.println(Thread.currentThread().getName() + " 进入 2"); 
				} 
			} 
		} else {
			synchronized (o2) { 
				System.out.println(Thread.currentThread().getName() + " 进入 3"); 
				synchronized (o1) {  
					System.out.println(Thread.currentThread().getName() + " 进入 4"); 
				} 
			} 
		} 
	} }

九、释放锁

1、下面操作会释放锁

当前线程的同步方法，同步代码块执行结束。
当前线程在同步代码块，同步方法中遇到break，return。
当前线程在同步代码块，同步方法中出现了未处理的错误或异常，导致异常结束。
当前线程在同步代码块，同步方法中执行的线程对象的wait方法，当前线程暂停，并释放锁。

2、 下面操作不会释放锁

线程执行同步代码块和同步方法时，程序调用Thread.sleep()，Thread.yield()方法暂停当前线程的执行，不会释放锁。
线程执行同步代码块时，其他线程调用了该线程的suspend()方法将该线程挂起，该线程不会释放锁。

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