synchronized同步方法（菜鸟玩线程）

怼烎@ 2024-02-19 12:59 45阅读 0赞

## synchronized同步方法 ##

### 方法内的变量是线程安全的 ###

#### 实验1： ####

##### 共享变量类 #####

package com.chapter02.thread1;
    
    public class HasSelfPrivateNum {
        
        public void addI(String username) {
        
            int num = 0;
            try {
        
                if (username.equals("a")) {
        
                    num = 100;
                    System.out.println("a set over!");
                    Thread.sleep(2000);
                } else {
        
                    num = 200;
                    System.out.println("b set over!");
                }
                System.out.println(username + " num = " + num);
            } catch (InterruptedException e) {
        
                e.printStackTrace();
            }
        }
    }

##### 自定义线程A #####

package com.chapter02.thread1;
    public class ThreadA extends Thread {
        
        private HasSelfPrivateNum hasSelfPrivateNum;
    
        public ThreadA(HasSelfPrivateNum hasSelfPrivateNum) {
        
            super();
            this.hasSelfPrivateNum = hasSelfPrivateNum;
        }
    
        @Override
        public void run() {
        
            super.run();
            hasSelfPrivateNum.addI("a");
        }
    }

##### 自定义线程B #####

package com.chapter02.thread1;
    public class ThreadB extends Thread {
        
        private HasSelfPrivateNum hasSelfPrivateNum;
        public ThreadB(HasSelfPrivateNum hasSelfPrivateNum) {
        
            super();
            this.hasSelfPrivateNum = hasSelfPrivateNum;
        }
        @Override
        public void run() {
        
            super.run();
            hasSelfPrivateNum.addI("b");
        }
    }

##### 启动类 #####

package com.chapter02.thread1;
    public class Run {
        
        public static void main(String[] args) {
        
            HasSelfPrivateNum hasSelfPrivateNum = new HasSelfPrivateNum();
            ThreadA threadA = new ThreadA(hasSelfPrivateNum);
            threadA.start();
            ThreadB threadB = new ThreadB(hasSelfPrivateNum);
            threadB.start();
        }
    }

##### 运行结果 #####

a set over!
    b set over!
    b num = 200
    a num = 100

##### 结论 #####

方法中的变量不存在非线程安全问题，永远都是线程安全的。

### 实例变量是非线程安全的 ###

#### 实验2： ####

##### 共享变量类 #####

package com.chapter02.thread1;
    
    public class HasSelfPrivateNum {
        
        //todo 注意此处 synchronized 和 变量
        private int num = 0;
        synchronized public void addI(String username) {
        
            try {
        
                if (username.equals("a")) {
        
                    num = 100;
                    System.out.println("a set over!");
                    Thread.sleep(2000);
                } else {
        
                    num = 200;
                    System.out.println("b set over!");
                }
                System.out.println(username + " num = " + num);
            } catch (InterruptedException e) {
        
                e.printStackTrace();
            }
        }
    }

##### 自定义线程A #####

##### 自定义线程B #####

package com.chapter02.thread1;
    public class ThreadB extends Thread {
        
        private HasSelfPrivateNum hasSelfPrivateNum;
    
        public ThreadB(HasSelfPrivateNum hasSelfPrivateNum) {
        
            super();
            this.hasSelfPrivateNum = hasSelfPrivateNum;
        }
    
        @Override
        public void run() {
        
            super.run();
            hasSelfPrivateNum.addI("b");
        }
    }

##### 启动类 #####

package com.chapter02.thread1;
    
    public class Run {
        
        public static void main(String[] args) {
        
    
            HasSelfPrivateNum hasSelfPrivateNum = new HasSelfPrivateNum();
            ThreadA threadA = new ThreadA(hasSelfPrivateNum);
            threadA.start();
    
            ThreadB threadB = new ThreadB(hasSelfPrivateNum);
            threadB.start();
    
        }
    }

##### 运行结果 #####

//如果不添加synchronized
    a set over!
    b set over!
    b num = 200
    a num = 200
    
    //如果添加synchronized
    a set over!
    a num = 100
    b set over!
    b num = 200

##### 结论 #####

在两个线程访问同一个对象中的同步方法时一定是线程安全的。

### 多个对象对个锁 ###

##### 修改实验2代码，修改启动类 #####

package com.chapter02.thread1;
    
    public class Run {
        
        public static void main(String[] args) {
        
    
            HasSelfPrivateNum hasSelfPrivateNumA = new HasSelfPrivateNum();
            ThreadA threadA = new ThreadA(hasSelfPrivateNumA);
            threadA.start();
    	    
            HasSelfPrivateNum hasSelfPrivateNumB = new HasSelfPrivateNum();
            ThreadB threadB = new ThreadB(hasSelfPrivateNumB);
            threadB.start();
    
        }
    }

##### 输出结果 #####

a set over!
    b set over!
    b num = 200
    a num = 100

##### 结论 #####

上面的示例 创建了2个HasSelfPrivateNum类的对象，所以产生了2个锁。