脏读（菜鸟玩线程）

野性酷女 2024-02-19 13:05 64阅读 0赞

## 脏读 ##

`脏读一定会出现在操作实例变量的情况下，这就是不同线程”争抢“实例变量的结果。`

### 脏读示例 ###

#### 定义共享变量实例类 ####

package com.chapter02.dirtyRead;
    
    public class PublicVar {
        
        public String username = "A";
        public String password = "AA";
    
        //添加synchronized
        synchronized public void setVaule(String username, String password) {
        
            try {
        
                this.username = username;
                Thread.sleep(5000);
                this.password = password;
                System.out.println("setValue method , thread name= " + Thread.currentThread().getName() +
                        ", username = " + username +
                        ", password = " + password);
            } catch (InterruptedException e) {
        
                e.printStackTrace();
            }
        }
    
        //无 synchronized
        public void getValue() {
        
            System.out.println("getValue method , thread name= " + Thread.currentThread().getName() +
                    ", username = " + username +
                    ", password = " + password);
        }
    }

#### 自定义线程类 ####

package com.chapter02.dirtyRead;
    public class ThreadA extends Thread {
        
        private PublicVar publicVar;
    
        public ThreadA(PublicVar publicVar) {
        
            this.publicVar = publicVar;
        }
    
        @Override
        public void run() {
        
            super.run();
            publicVar.setVaule("B", "BB");
        }
    }

#### 启动类 ####

package com.chapter02.dirtyRead;
    
    public class TestRun {
        
        public static void main(String[] args) {
        
            try {
        
                PublicVar publicVar = new PublicVar();
                ThreadA threadA = new ThreadA(publicVar);
                threadA.start();
                Thread.sleep(2000); //打印结果受此值的影响
                publicVar.getValue();
            } catch (Exception ex) {
        
                ex.printStackTrace();
            }
        }
    }

#### 执行结果 ####

//出现脏读
    getValue method , thread name= main, username = B, password = AA
    setValue method , thread name= Thread-0, username = B, password = BB

#### 总结 ####

`当A线程调用PublicVar对象加入synchronized关键字的setVaule()方法时，A线程就获得了setVaule()方法的锁，更准确的讲，是获得了对象的锁，所以其他线程必须等到A线程执行完毕才可以调用setVaule()方法，但B线程可以随意调用其他的非synchronized方法。`

### 防止脏读示例 ###

#### 修改 ####

修改上面“定义共享变量实例类PublicVar ”中的getValue()方法，增加synchronized关键字，如下：

package com.chapter02.dirtyRead;
    
    public class PublicVar {
        
        public String username = "A";
        public String password = "AA";
    
        //添加synchronized
        synchronized public void setVaule(String username, String password) {
        
            try {
        
                this.username = username;
                Thread.sleep(5000);
                this.password = password;
                System.out.println("setValue method , thread name= " + Thread.currentThread().getName() +
                        ", username = " + username +
                        ", password = " + password);
            } catch (InterruptedException e) {
        
                e.printStackTrace();
            }
        }
    
        //有 synchronized
        synchronized public void getValue() {
        
            System.out.println("getValue method , thread name= " + Thread.currentThread().getName() +
                    ", username = " + username +
                    ", password = " + password);
        }
    }

#### 执行结果 ####

setValue method , thread name= Thread-0, username = B, password = BB
    getValue method , thread name= main, username = B, password = BB

#### 总结 ####

`当A线程调用PublicVar对象加入synchronized关键字的setVaule()方法时，A线程就获得了setValue()方法所在对象的锁，所以其他线程必须等A线程执行完毕才可以调用setValue()方法，而B线程如果调用声明了synchronized关键字的getValue()方法时，必须等A线程将setValue()方法执行完，也就是释放对象锁后才可以调用。这时A线程执行了一个完整的任务，也就是说username和password这两个实例变量已经同时被赋值，不存在脏读的基本环境。`