synchronized将任意对象作为对象监控器（菜鸟玩线程）

曾经终败给现在 2024-02-19 13:09 61阅读 0赞

## 将任意对象作为对象监控器 ##

多线程调用同一个对象中的不通名称的synchronized同步方法或synchronized(this)同步代码块时，调用的效果就是按顺序执行，也就是同步的，阻塞的。

### (1.1)理论：synchronized(非this)… ###

锁非this对象具有一定的有优点：如果一个类中有很多个synchronized方法，这时虽然能实现同步，但会受到阻塞，所以影响运行效率；但如果使用同步代码块锁非this对象，则synchronized(非this)代码块中的程序与同步方法是异步的，不与其他锁this同步方法争抢this锁，则可大大提高运行效率。

### (1.2)落地论证 ###

#### (1.2.1)自定义共享变量类 ####

package com.chapter02.thread2_2_7_2;
    
    public class Service {
        
        private String anyString = new String();
        public void a() {
        
            try {
        
                //synchronized(非this)锁
                synchronized (anyString) {
        
                    System.out.println("A BEGIN!");
                    Thread.sleep(3000);
                    System.out.println("A END");
                }
            } catch (Exception e) {
        
                e.printStackTrace();
            }
        }
    
        //synchronized(this)锁
        synchronized public void b() {
        
            System.out.println("B BEGIN!");
            System.out.println("B END!");
        }
    }

#### (1.2.2)自定义线程1 ####

package com.chapter02.thread2_2_7_2;
    public class ThreadA extends Thread {
        
        private Service service;
    
        public ThreadA(Service service) {
        
            this.service = service;
        }
    
        @Override
        public void run() {
        
            super.run();
            service.a();
        }
    }

#### (1.2.3)自定义线程2 ####

package com.chapter02.thread2_2_7_2;
    
    public class ThreadB extends Thread {
        
        private Service service;
    
        public ThreadB(Service service) {
        
            this.service = service;
        }
    
        @Override
        public void run() {
        
            super.run();
            service.b();
        }
    }

#### (1.2.4)启动类 ####

package com.chapter02.thread2_2_7_2;
    
    public class Run {
        
        public static void main(String[] args) {
        
            Service service = new Service();
    
            ThreadA threadA = new ThreadA(service);
            threadA.setName("A");
            threadA.start();
    
            ThreadB threadB = new ThreadB(service);
            threadB.setName("B");
            threadB.start();
        }
    }

#### (1.2.5)执行结果 ####

A BEGIN!
    B BEGIN!
    B END!
    A END

#### (1.2.6)结论 ####

`synchronized(非this)代码块中的程序与同步方法是异步的，不与其他锁this同步方法争抢this锁`

### (2.1)synchronized(非this)出现脏读情况 ###

#### （2.1.1）理论 ####

`一个线程握一把自己的锁，就会出现脏读情况。如果多个线程使用同一把锁，就会避免脏读。`

#### （2.1.2）落地论证 ####

##### 自定义共享变量类 #####

package com.chapter02.thread2_2_7_3;
    
    import java.util.ArrayList;
    import java.util.List;
    
    public class MyOneList {
        
        private List list = new ArrayList();
    
        synchronized public void add(String data) {
        
            list.add(data);
        }
    
        synchronized public int getSize() {
        
            return list.size();
        }
    }

package com.chapter02.thread2_2_7_3;
    
    public class MyService {
        
        public MyOneList addServiceMethod(MyOneList list, String data) {
        
            try {
        
                if (list.getSize() < 1) {
        
                    Thread.sleep(2000);
                    list.add(data);
                }
            } catch (Exception e) {
        
                e.printStackTrace();
            }
            return list;
        }
    }

##### 自定义线程1 #####

package com.chapter02.thread2_2_7_3;
    
    public class MyThread1 extends Thread {
        
        private MyOneList myOneList;
    
        public MyThread1(MyOneList myOneList) {
        
            this.myOneList = myOneList;
        }
    
        @Override
        public void run() {
        
            MyService myService = new MyService();
            myService.addServiceMethod(myOneList, "A");
        }
    }

##### 自定义线程2 #####

package com.chapter02.thread2_2_7_3;
    
    public class MyThread2 extends Thread {
        
        private MyOneList myOneList;
    
        public MyThread2(MyOneList myOneList) {
        
            this.myOneList = myOneList;
        }
    
        @Override
        public void run() {
        
            MyService myService = new MyService();
            myService.addServiceMethod(myOneList, "B");
        }
    }

##### 启动类 #####

package com.chapter02.thread2_2_7_3;
    
    public class Run {
        
        public static void main(String[] args) {
        
            MyOneList list = new MyOneList();
    
            MyThread1 myThread1 = new MyThread1(list);
            myThread1.setName("A");
            myThread1.start();
    
            MyThread2 myThread2 = new MyThread2(list);
            myThread2.setName("B");
            myThread2.start();
    
            try {
        
                Thread.sleep(6000);
            } catch (InterruptedException e) {
        
                e.printStackTrace();
            }
            System.out.println("LIST SIZE : " + list.getSize());
        }
    }

##### 执行结果 #####

//出现脏读，正确因该为1
    LIST SIZE : 2

#### （2.1.3）避免脏读 ####

##### 修改MyService类 #####

如下：

package com.chapter02.thread2_2_7_3;
    
    public class MyService {
        
        public MyOneList addServiceMethod(MyOneList list, String data) {
        
            try {
        
                synchronized (list){
        
                    if (list.getSize() < 1) {
        
                        Thread.sleep(2000);
                        list.add(data);
                    }
                }
            } catch (Exception e) {
        
                e.printStackTrace();
            }
            return list;
        }
    
    }

##### 执行结果 #####

//结果正常
    LIST SIZE : 1