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synchronized是一个重量级锁,我们都知道该关键字锁住的是对象而不是代码本身,那么对于静态方法和同步方法有什么不同呢,通过如下代码进行测试
public class SynchronizedTest {
private static int num;
private synchronized void test(String param){
if(StringUtils.equals(param,"a")){
num = 100;
System.out.println("set " + param + " num over");
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}else{
num = 200;
System.out.println("set " + param + " num over");
}
System.out.println("i am : " + param + "; num = " + num);
}
public static void main(String[] args){
SynchronizedTest s1 = new SynchronizedTest();
SynchronizedTest s2 = new SynchronizedTest();
new Thread(new Runnable() {
@Override
public void run() {
s1.test("a");
}
}).start();
new Thread(new Runnable() {
@Override
public void run() {
s2.test("b");
}
}).start();
}
}
输出结果为:
set a num over
set b num over
i am : b; num = 200
i am : a; num = 200
Process finished with exit code 0
我们可以看出两个不同的对象s1和s2并没有互斥,因为这里synchronized是分别持有两个对象的锁。如果要想m1,m2两个对象竞争同一个锁,则需要在method01()上加上static修饰,如下:
public class SynchronizedTest {
private static int num;
private synchronized static void test(String param){
if(StringUtils.equals(param,"a")){
num = 100;
System.out.println("set " + param + " num over");
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}else{
num = 200;
System.out.println("set " + param + " num over");
}
System.out.println("i am : " + param + "; num = " + num);
}
public static void main(String[] args){
SynchronizedTest s1 = new SynchronizedTest();
SynchronizedTest s2 = new SynchronizedTest();
new Thread(new Runnable() {
@Override
public void run() {
s1.test("a");
}
}).start();
new Thread(new Runnable() {
@Override
public void run() {
s2.test("b");
}
}).start();
}
}
运行结果如下:
set a num over
i am : a; num = 100
set b num over
i am : b; num = 200
Process finished with exit code 0
synchronized修饰不加static的方法,锁是加在单个对象上,不同的对象没有竞争关系;修饰加了static的方法,锁是加载类上,这个类所有的对象竞争一把锁.