Synchronized locked objects

Preface

Now I'm going to go over it again 《Java The art of concurrent programming 》, There is a passage as follows ：

Java Each object in can be used as a lock . Specific for ：

• Common synchronization method , The lock is the current instance object .
• Static synchronization method , The lock is for the current class Class object .
• Synchronized method block , A lock is an object configured in parentheses .

Has not been verified , Verify today .

Text

The test code is simple , Just print a sentence , Then sleep 5s. branch 3 Look at the different dump Information .

Common synchronization method

import java.util.concurrent.TimeUnit;
public class Test {
    
    public static void main(String[] args) {
    
        Test test = new Test();
        test.helper();
    }

    public synchronized void helper() {
    
        try {
    
            System.out.println("hello");
            TimeUnit.SECONDS.sleep(5);
        } catch (InterruptedException e) {
    
            e.printStackTrace();
        }
    }
}

result ：

"main" #1 prio=5 os_prio=0 tid=0x00c46800 nid=0x754 waiting on condition [0x0283f000]
   java.lang.Thread.State: TIMED_WAITING (sleeping)
	at java.lang.Thread.sleep(Native Method)
	at java.lang.Thread.sleep(Thread.java:340)
	at java.util.concurrent.TimeUnit.sleep(TimeUnit.java:386)
	at com.company.Test.helper(Test.java:16)
	- locked <0x04b45e20> (a com.company.Test)
	at com.company.Test.main(Test.java:10)

The key is ：- locked <0x04b45e20> (a com.company.Test)

Static synchronization method

import java.util.concurrent.TimeUnit;

public class Test {
    
    public static void main(String[] args) {
    
        Test test = new Test();
        test.helper();
    }

    public static synchronized void helper() {
    
        try {
    
            System.out.println("hello");
            TimeUnit.SECONDS.sleep(5);
        } catch (InterruptedException e) {
    
            e.printStackTrace();
        }
    }
}

result ：

"main" #1 prio=5 os_prio=0 tid=0x013d5c00 nid=0x1b28 waiting on condition [0x02edf000]
   java.lang.Thread.State: TIMED_WAITING (sleeping)
	at java.lang.Thread.sleep(Native Method)
	at java.lang.Thread.sleep(Thread.java:340)
	at java.util.concurrent.TimeUnit.sleep(TimeUnit.java:386)
	at com.company.Test.helper(Test.java:14)
	- locked <0x05143378> (a java.lang.Class for com.company.Test)
	at com.company.Test.main(Test.java:8)

The key is ：- locked <0x05143378> (a java.lang.Class for com.company.Test)

Synchronized block

import java.util.concurrent.TimeUnit;

public class Test {
    
    public static String lock = "";
    public static void main(String[] args) {
    
        Test test = new Test();
        test.helper();
    }

    public static void helper() {
    
        synchronized (lock) {
    
            try {
    
                System.out.println("hello");
                TimeUnit.SECONDS.sleep(5);
            } catch (InterruptedException e) {
    
                e.printStackTrace();
            }
        }
    }
}

result ：

"main" #1 prio=5 os_prio=0 tid=0x01336000 nid=0x258c waiting on condition [0x0164f000]
   java.lang.Thread.State: TIMED_WAITING (sleeping)
	at java.lang.Thread.sleep(Native Method)
	at java.lang.Thread.sleep(Thread.java:340)
	at java.util.concurrent.TimeUnit.sleep(TimeUnit.java:386)
	at com.company.Test.helper(Test.java:16)
	- locked <0x050095e0> (a java.lang.String)
	at com.company.Test.main(Test.java:9)

The key is ：- locked <0x050095e0> (a java.lang.String)

Conclusion

Through these three examples, we can verify the contents of the book ：

form	The object of the lock
Synchronization method	a com.company.Test
Static synchronization method	a java.lang.Class for com.company.Test
A static block （ In brackets is String object ）	a java.lang.String