synchronized的一些理解

1. 两个普通的synchronized

package ThreadTest;

import java.util.concurrent.TimeUnit;

public class ThreadTest02 {
    public static void main(String[] args) {
        Phone phone = new Phone();
        new Thread(phone::sendMsg, "A").start();
        try {
            TimeUnit.SECONDS.sleep(1);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        new Thread(phone::call, "B").start();
    }
}

class Phone{
    public synchronized void sendMsg(){
        try {
            TimeUnit.SECONDS.sleep(4);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("Send message!");
    }
    public synchronized void call(){
        System.out.println("Call");
    }
}

这段代码的执行顺序是：

线程A获得phone的锁等待4s，同时main等待1s，因为1s小于4s，所以4s过后线程A打印发信息的函数，瞬间释放锁，线程B拿到锁执行call

2. 两个带有static的同步方法

package ThreadTest;

import java.util.concurrent.TimeUnit;

public class ThreadTest02 {
    public static void main(String[] args) {
        Phone phone1 = new Phone();
        Phone phone2 = new Phone();
        new Thread(()->{
            phone1.sendMsg();
        }, "A").start();
        try {
            TimeUnit.SECONDS.sleep(1);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        new Thread(()->{
            phone2.call();
        }, "B").start();
    }
}

class Phone{
    public static synchronized void sendMsg(){
        try {
            TimeUnit.SECONDS.sleep(4);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("Send message!");
    }
    public static synchronized void call(){
        System.out.println("Call");
    }
}

这段代码实例化两个对象，但是因为两个同步方法是static的，所以是类锁，效果和上面那个一样

3.一个普通的synchorized和一个带有static的同步方法

package ThreadTest;

import java.util.concurrent.TimeUnit;

public class ThreadTest02 {
    public static void main(String[] args) {
        Phone phone1 = new Phone();
        Phone phone2 = new Phone();
        new Thread(()->{
            phone1.sendMsg();
        }, "A").start();
        try {
            TimeUnit.SECONDS.sleep(1);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        new Thread(()->{
            phone2.call();
        }, "B").start();
    }
}

class Phone{
    public static synchronized void sendMsg(){
        try {
            TimeUnit.SECONDS.sleep(4);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("Send message!");
    }
    public synchronized void call(){
        System.out.println("Call");
    }
}

现在把下面的static删掉，call方法会先执行，因为有两个锁，一个是类锁，一个是对象锁，所以互不影响。