synchronized 对象锁，采用互斥的方式让同一时刻至多有一个线程能持有「对象锁」。
其他锁想要获取这个「对象锁」就会被阻塞住。
需要注意
- 互斥是保证临界区的竞态条件发生，同一时刻只能有一个线程执行临界区代码
- 同步是由于线程的执行顺序
对象锁
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| package com.redisc;
import lombok.extern.slf4j.Slf4j;
@Slf4j(topic = "c.Test")
public class Test {
static int counter = 0;
static Object Lock = new Object();
public static void main(String[] args) throws InterruptedException {
Thread t1 = new Thread(() -> {
for (int i = 0; i < 5000; i++) {
synchronized (Lock) {
counter++;
}
}
});
Thread t2 = new Thread(() -> {
for (int i = 0; i < 5000; i++) {
synchronized (Lock) {
counter--;
}
}
});
t1.start();
t2.start();
t1.join();
t2.join();
log.debug("{}", counter);
}
}
|
输出
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| 17:45:38.434 [main] DEBUG c.Test - 0
|
使用 synchronized 加锁对象后，任何一个线程在处理临界对象的时候，都需要先请求对象锁。
synchronized 实际上是用对象锁保证了临界区内代码的原子性。临界区内的代码对外是不可分隔的。
面向对象的锁的改进
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| package com.redisc;
import lombok.extern.slf4j.Slf4j;
@Slf4j(topic = "c.Test")
public class Test {
public static void main(String[] args) throws InterruptedException {
Room room = new Room();
Thread t1 = new Thread(() -> {
for (int i = 0; i < 5000; i++) {
room.decrement();
}
});
Thread t2 = new Thread(() -> {
for (int i = 0; i < 5000; i++) {
room.increment();
}
});
t1.start();
t2.start();
t1.join();
t2.join();
log.debug("{}", room.getCounter());
}
}
class Room {
private int counter = 0;
public void increment() {
synchronized (this) {
counter++;
}
}
public void decrement() {
synchronized (this) {
counter--;
}
}
public int getCounter() {
synchronized (this) {
return this.counter;
}
}
}
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输出
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| 21:51:10.538 [main] DEBUG c.Test - 0
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加在方法
对象方法
加在对象方法上，锁定的是 this。
语法如下
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| class Test{
public synchronized void test(){
}
}
等价于
class Test{
public void test(){
synchronized(this){
}
}
}
|
例子如下
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| package com.redisc;
import lombok.extern.slf4j.Slf4j;
@Slf4j(topic = "c.Test")
public class Test {
public static void main(String[] args) throws InterruptedException {
Room room = new Room();
Thread t1 = new Thread(() -> {
for (int i = 0; i < 5000; i++) {
room.decrement();
}
});
Thread t2 = new Thread(() -> {
for (int i = 0; i < 5000; i++) {
room.increment();
}
});
t1.start();
t2.start();
t1.join();
t2.join();
log.debug("{}", room.getCounter());
}
}
class Room {
private int counter = 0;
public synchronized void increment() {
counter++;
}
public synchronized void decrement() {
counter--;
}
public synchronized int getCounter() {
return this.counter;
}
}
|
输出
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| 21:55:41.435 [main] DEBUG c.Test - 0
|
静态方法
语法
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| class Test{
public synchronized static void test(){
}
}
等价于
class Test{
public static void test(){
synchronized(Test.class){
}
}
}
|
