髒讀

髒讀一定會出現在操作例項變數的情況下，這就是不同執行緒”爭搶“例項變數的結果。

髒讀示例

定義共享變數例項類

package com.chapter02.dirtyRead;

public class PublicVar {
    public String username = "A";
    public String password = "AA";

    //新增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();
        }
    }

    //無 synchronized
    public void getValue() {
        System.out.println("getValue method , thread name= " + Thread.currentThread().getName() +
                ", username = " + username +
                ", password = "
 
 + password);
    }
}

自定義執行緒類

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");
    }
}

啟動類

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); //列印結果受此值的影響
            publicVar.getValue();
        } catch (Exception ex) {
            ex.printStackTrace();
        }
    }
}

執行結果

//出現髒讀
getValue method , thread name= main, username = B, password = AA
setValue method , thread name= Thread-0, username = B, password = BB

總結

當A執行緒呼叫PublicVar物件加入synchronized關鍵字的setVaule()方法時，A執行緒就獲得了setVaule()方法的鎖，更準確的講，是獲得了物件的鎖，所以其他執行緒必須等到A執行緒執行完畢才可以呼叫setVaule()方法，但B執行緒可以隨意呼叫其他的非synchronized方法。

防止髒讀示例

修改

修改上面“定義共享變數例項類PublicVar ”中的getValue()方法，增加synchronized關鍵字，如下：

package com.chapter02.dirtyRead;

public class PublicVar {
    public String username = "A";
    public String password = "AA";

    //新增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);
    }
}

執行結果

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

總結

當A執行緒呼叫PublicVar物件加入synchronized關鍵字的setVaule()方法時，A執行緒就獲得了setValue()方法所在物件的鎖，所以其他執行緒必須等A執行緒執行完畢才可以呼叫setValue()方法，而B執行緒如果呼叫聲明瞭synchronized關鍵字的getValue()方法時，必須等A執行緒將setValue()方法執行完，也就是釋放物件鎖後才可以呼叫。這時A執行緒執行了一個完整的任務，也就是說username和password這兩個例項變數已經同時被賦值，不存在髒讀的基本環境。