keyword synchronized It can also be applied to static Statically , If you write like this , That's right now .java File corresponding Class Class to lock .*
Here is an example to illustrate the usage and effect .
(1) Service.java
public class Service {
synchronized public static void printA() {
try {
System.out.println(" The thread name is ：" + Thread.currentThread().getName()
+ " stay " + System.currentTimeMillis() + " Get into printA");
Thread.sleep(3000);
System.out.println("-- The thread name is ：" + Thread.currentThread().getName()
+ " stay " + System.currentTimeMillis() + " Leave printA");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
synchronized public static void printB() {
System.out.println(" The thread name is ：" + Thread.currentThread().getName() + " stay "
+ System.currentTimeMillis() + " Get into printB");
System.out.println(" The thread name is ：" + Thread.currentThread().getName() + " stay "
+ System.currentTimeMillis() + " Leave printB");
}
}
(2) ThreadA.java
public class ThreadA extends Thread {
@Override
public void run() {
Service.printA();
}
}
(3) ThreadB.java
public class ThreadB extends Thread {
@Override
public void run() {
Service.printB();
}
}
(4) Run.java
public class Run {
public static void main(String[] args) {
ThreadA a = new ThreadA();
a.setName("A");
a.start();
ThreadB b = new ThreadB();
b.setName("B");
b.start();
}
}
The thread name is ：A stay 1634056855437 Get into printA
-- The thread name is ：A stay 1634056858458 Leave printA
The thread name is ：B stay 1634056858458 Get into printB
The thread name is ：B stay 1634056858459 Leave printB
From the results of operation , Nothing special , It's all synchronous , And will be synchronized Keywords added to non static The effect of the method is the same . In fact, there are essential differences ,synchronized Keywords added to static Static method is to give Class Class lock , and synchronized Keywords added to non static Static methods are object locking .
To prove that it's not the same lock , We can verify it through the following experiments ：
(1) Service.java
public class Service {
synchronized public static void printA() {
try {
System.out.println(" The thread name is ：" + Thread.currentThread().getName()
+ " stay " + System.currentTimeMillis() + " Get into printA");
Thread.sleep(3000);
System.out.println("---- The thread name is ：" + Thread.currentThread().getName()
+ " stay " + System.currentTimeMillis() + " Leave printA");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
synchronized public static void printB() {
System.out.println(" The thread name is ：" + Thread.currentThread().getName() + " stay "
+ System.currentTimeMillis() + " Get into printB");
System.out.println(" The thread name is ：" + Thread.currentThread().getName() + " stay "
+ System.currentTimeMillis() + " Leave printB");
}
synchronized public void printC() {
System.out.println(" The thread name is ：" + Thread.currentThread().getName() + " stay "
+ System.currentTimeMillis() + " Get into printC");
System.out.println("???? The thread name is ：" + Thread.currentThread().getName() + " stay "
+ System.currentTimeMillis() + " Leave printC");
}
}
(2) ThreadA.java
public class ThreadA extends Thread {
private Service service;
public ThreadA(Service service) {
super();
this.service = service;
}
@Override
public void run() {
service.printA();
}
}
(3) ThreadB.java
public class ThreadB extends Thread {
private Service service;
public ThreadB(Service service) {
super();
this.service = service;
}
@Override
public void run() {
service.printB();
}
}
(4) ThreadC.java
public class ThreadC extends Thread {
private Service service;
public ThreadC(Service service) {
super();
this.service = service;
}
@Override
public void run() {
service.printC();
}
}
(5) Run.java
public class Run {
public static void main(String[] args) {
Service service = new Service();
ThreadA a = new ThreadA(service);
a.setName("A");
a.start();
ThreadB b = new ThreadB(service);
b.setName("B");
b.start();
ThreadC c = new ThreadC(service);
c.setName("C");
c.start();
}
}
Execution results ：
The thread name is ：A stay 1634056663156 Get into printA
The thread name is ：C stay 1634056663157 Get into printC
???? The thread name is ：C stay 1634056663185 Leave printC
---- The thread name is ：A stay 1634056666186 Leave printA
The thread name is ：B stay 1634056666186 Get into printB
The thread name is ：B stay 1634056666186 Leave printB
The experimental results show that ： This experiment actually sets up two locks ：
- Object lock ：printC Yes no static method , Constrained by object locks
- Class lock ：PrintA、PrintB It's a static method , All by the same Class constraint
The above can also be Service.java In code PrintB Change it to ：
synchronized public void printB() {
System.out.println(" The thread name is ：" + Thread.currentThread().getName() + " stay "
+ System.currentTimeMillis() + " Get into printB");
try {
Thread.sleep(5000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(" The thread name is ：" + Thread.currentThread().getName() + " stay "
+ System.currentTimeMillis() + " Leave printB");
}
Rerun , give the result as follows :
The thread name is ：B stay 1634058460652 Get into printB
The thread name is ：A stay 1634058460652 Get into printA
---- The thread name is ：A stay 1634058461681 Leave printA
The thread name is ：B stay 1634058465681 Leave printB
The thread name is ：C stay 1634058465681 Get into printC
???? The thread name is ：C stay 1634058465682 Leave printC
PrintB and PrintC Are non static , Belong to the same lock , therefore C Waiting B Release the lock when finished .
The reason for asynchrony is to hold different locks , One is object lock , The other is Class lock , and Class Locks work on all object instances of the class .
Sync synchronized(class) The function of code block is actually the same as synchronized static The method works the same .
With the following example （ Omit Thread file ） Verify that :
(1) Service.java
public class Service {
public static void printA() {
synchronized (Service.class) {
try {
System.out.println(" The thread name is ：" + Thread.currentThread().getName()
+ " stay " + System.currentTimeMillis() + " Get into printA");
Thread.sleep(3000);
System.out.println("---- The thread name is ：" + Thread.currentThread().getName()
+ " stay " + System.currentTimeMillis() + " Leave printA");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public static void printB() {
synchronized (Service.class) {
System.out.println(" The thread name is ：" + Thread.currentThread().getName()
+ " stay " + System.currentTimeMillis() + " Get into printB");
System.out.println("++++ The thread name is ：" + Thread.currentThread().getName()
+ " stay " + System.currentTimeMillis() + " Leave printB");
}
}
}
(2)Run.java
public class Run {
public static void main(String[] args) {
Service service1 = new Service();
Service service2 = new Service();
ThreadA a = new ThreadA(service1);
a.setName("A");
a.start();
ThreadB b = new ThreadB(service2);
b.setName("B");
b.start();
}
}
Execution results :
The thread name is ：A stay 1634098725887 Get into printA
---- The thread name is ：A stay 1634098728911 Leave printA
The thread name is ：B stay 1634098728911 Get into printB
++++ The thread name is ：B stay 1634098728912 Leave printB