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Java中常用的两个锁synchronized和ReentrantLock,那么它们在原理和使用上有什么区别呢?
二者的区别
1.原始构成
synchronized是关键字属于JVM层面
monitorenter
底层通过monitor对象来完成,其实wait/notify等方法也依赖于monitor对象只有在同步块或方法中才能调wait/notify等方法。
monitorexit
Lock是具体类()是api层面的锁
public class Code02_Demo {
public static void main(String[] args) {
synchronized (new Object()){
}
new ReentrantLock();
}
}
javap-c得到
可以看到synchronized有两个monitorexit是因为第一个monitorexit是正常情况下的解锁,而第二个monitorexit是异常情况下的解锁,这样不会出现死锁
可以看到ReentrantLock是个类
2.使用方法
synchronized不需要用户去手动释放锁,当synchronized代码执行完后系统会自动让线程释放对锁的占用。
需要lock,unlock配合try finally完成
3.等待是否可中断
使用方法synchronized不可中断,除非抛出异常或者正常运行完成,
ReentrantLock可以中断,
1.设置超时方法tryLock(long time, TimeUnit unit)
2.lockInterruptibly()放代码块中,调用interrupt()方法可中断
4.加锁是否公平
synchronized默认是非公平锁
ReentrantLock默认也是非公平锁
5.锁绑定多个条件Condition
ReentrantLock可以分组唤醒线程,而不是像synchronized那样要么全部唤醒,要么随机唤醒一个。
题目
A线程打印1次,B线程打印2次,C线程打印3次,循环5次
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class Code01_ReentrantLockDemo {
public static void main(String[] args) {
ResourceInfo resourceInfo = new ResourceInfo();
new Thread(() -> {
for (int i = 1; i <= 5; i++) {
resourceInfo.print1();
}
}, "A").start();
new Thread(() -> {
for (int i = 1; i <= 5; i++) {
resourceInfo.print2();
}
}, "B").start();
new Thread(() -> {
for (int i = 1; i <= 5; i++) {
resourceInfo.print3();
}
}, "C").start();
}
}
class ResourceInfo {
Lock lock = new ReentrantLock();
private int num = 1; //标志变量num
Condition c1 = lock.newCondition();
Condition c2 = lock.newCondition();
Condition c3 = lock.newCondition();
public void print1() {
lock.lock();
try {
while (num != 1) {
c1.await();
}
System.out.println(Thread.currentThread().getName() + "\t");
//唤醒B线程,修改标志位
num = 2;
c2.signal();
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public void print2() {
lock.lock();
try {
while (num != 2) {
c2.await();
}
for (int i = 1; i <= 2; i++) {
System.out.println(Thread.currentThread().getName() + "\t");
}
//唤醒B线程,修改标志位
num = 3;
c3.signal();
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public void print3() {
lock.lock();
try {
while (num != 3) {
c3.await();
}
for (int i = 1; i <= 3; i++) {
System.out.println(Thread.currentThread().getName() + "\t");
}
//唤醒B线程,修改标志位
num = 1;
c1.signal();
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
}