Synchronized和ReentrantLock的区别
- 原始构成:synchronized是关键字,属于JVM层面,底层通过monitor对象来完成,wait/notify等方法也依赖于monitor,只有在同步块或方法中才能调用wait/notify等方法;Lock是个类,是api层面的锁。
- 使用方法:synchronized 不需要手动释放锁,当synchronized 代码执行完后系统会自动让线程释放对锁的占用;Lock需要手动开锁和释放锁,不释放则会造成死锁。通过lock.lock()和lock.unlock()结合try...catch...finally语句块来完成。
- 等待是否可中断:synchronized 不可中断,除非运行完毕或异常中断;Lock可中断,设置超时方法tryLock(long timeout,TimeUnit unit),调用 interrupt() 方法中断
- 加锁是否公平:synchronized 是非公平锁;ReentrantLock 既可以是公平锁,也可以是非公平锁,根据参数设置
- 锁绑定多个条件Condition:synchronized 没有这个功能,ReentrantLock 用来实现分组唤醒需要唤醒的线程们,可以精确唤醒,而不是像 synchronized 似的随即唤醒一个或者全部唤醒
ReentrantLock 如何精确唤醒线程
设定场景:三个线程按顺序调用,实现线程A→B→C依次启动,其中A打印 1 次,B打印 2 次,C打印 3 次,再如此循环 3 次
class ShareRes{
private int num = 1; //A:1,B:2,C:3
private ReentrantLock lock = new ReentrantLock();
private Condition conditionA = lock.newCondition();
private Condition conditionB = lock.newCondition();
private Condition conditionC = lock.newCondition();
void print1(){
lock.lock();
try {
while (num != 1){
conditionA.await();
}
for (int i = 1; i <= 1; i++) {
System.out.println(Thread.currentThread().getName()+" "+i);
}
num = 2;
conditionB.signal();
}catch (Exception e){
e.printStackTrace();
}finally {
lock.unlock();
}
}
void print2(){
lock.lock();
try {
while (num != 2){
conditionB.await();
}
for (int i = 1; i <= 2; i++) {
System.out.println(Thread.currentThread().getName()+" "+i);
}
num = 3;
conditionC.signal();
}catch (Exception e){
e.printStackTrace();
}finally {
lock.unlock();
}
}
void print3(){
lock.lock();
try {
while (num != 3){
conditionC.await();
}
for (int i = 1; i <= 3; i++) {
System.out.println(Thread.currentThread().getName()+" "+i);
}
num = 1;
conditionA.signal();
}catch (Exception e){
e.printStackTrace();
}finally {
lock.unlock();
}
}
}
public class LockTraditionDemo {
public static void main(String[] args) {
final ShareRes res = new ShareRes();
new Thread(new Runnable() {
@Override
public void run() {
for (int i = 0; i < 3; i++) {
res.print1();
}
}
},"线程A").start();
new Thread(new Runnable() {
@Override
public void run() {
for (int i = 0; i < 3; i++) {
res.print2();
}
}
},"线程B").start();
new Thread(new Runnable() {
@Override
public void run() {
for (int i = 0; i < 3; i++) {
res.print3();
}
}
},"线程C").start();
}
}
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线程A 1
线程B 1
线程B 2
线程C 1
线程C 2
线程C 3
线程A 1
线程B 1
线程B 2
线程C 1
线程C 2
线程C 3
线程A 1
线程B 1
线程B 2
线程C 1
线程C 2
线程C 3
ReentrantLock 绑定了三个条件对象,可以精确唤醒线程。