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多线程-synchronized锁在类级别和方法级执行结果的区别

2018-01-15 00:00 316 查看

synchronized 方法级别代码：

public class Thread1 {
private int num = 0;
public synchronized void printNum(String arg){

if (arg.equals("a")){
num = 100;
System.out.println("num value is "+ num);
try {
new Thread().sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}else {
num = 200;
System.out.println("num value is "+ num);
}
System.out.println("tag = " + arg + " num = " + num);
}

public static void main(String[] args) {
final Thread1 t1 = new Thread1();
final Thread1 t2 = new Thread1();

Thread thread1 = new Thread(new Runnable() {
@Override
public void run() {
t1.printNum("a");
}
});

Thread thread2 = new Thread(new Runnable() {
@Override
public void run() {
t2.printNum("b");
}
});

thread1.start();
thread2.start();
}
}

执行结果：

num value is 200
tag = b num = 200
num value is 100
tag = a num = 100

synchronized 类级别代码：

public class Thread1 {
private static int num = 0;
public static synchronized void printNum(String arg){

if (arg.equals("a")){
num = 100;
System.out.println("num value is "+ num);
try {
new Thread().sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}else {
num = 200;
System.out.println("num value is "+ num);
}
System.out.println("tag = " + arg + " num = " + num);
}

public static void main(String[] args) {
final Thread1 t1 = new Thread1();
final Thread1 t2 = new Thread1();

Thread thread1 = new Thread(new Runnable() {
@Override
public void run() {
t1.printNum("a");
}
});

Thread thread2 = new Thread(new Runnable() {
@Override
public void run() {
t2.printNum("b");
}
});

thread1.start();
thread2.start();
}
}

执行结果：

num value is 100
tag = a num = 100
num value is 200
tag = b num = 200

总结：

关键字synchronized取得的锁都试对象锁，而不是把一段代码（方法）当做锁，所以十里代码中哪个线程限制性synchronized关键字的方法，哪个线程就持有该方法所属对象的锁（Lock），两个对象，线程获得的就是两个不通的锁，他们互补影响。
有一种情况则是相同的锁，即在静态方法上加synchronized关键字，表示锁定.class类，类一级别的锁（独占.class类）。

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