公平锁与非公平锁
公平锁：多个线程按照申请锁的顺序来获取锁 类似食堂排队，先来后到
非公平锁：是指多个线程获取锁的顺序并不是按照申请锁的顺序，有可能后申请的线程比先申请的线程先拿到锁，在高并发的情况下有可能造成优先级反转和饥饿现象
并发包中 ReentrantLock 可以创建公平锁和非公平锁 默认情况是非公平锁，Sychnoized也是非公平锁，非公平锁的有点是吞吐量大
当一个线程想获取锁时，先试图插队，如果占用锁的线程释放了锁，下一个线程还没来得及拿锁，那么当前线程就可以直接获得锁；如果锁正在被其它线程占用，则排队，排队的时候就不能再试图获得锁了，只能等到前面所有线程都执行完才能获得锁
可重入锁（可以避免死锁）
ReentrantLock 可重入锁（递归锁）：同一个线程外层函数获取锁之后，内层函数依然能够获取该锁的代码
在同一个线程在外层方法获取锁的时候，进入内层方法会自动获取该锁
线程可以进入一个已经拥有锁所同步的代码块
Sychnoized和ReentrantLock 默认情况是是非公平的重入锁
后面一个demo会详细的解释这种情况
先来看下Sychnoized 重入锁的情况
package com.zz.study.lock;
public class TestSynReentry {
public static void main(String[] args) {
new Thread( () -> {
SendWarningPush();
},"one").start();
new Thread( () -> {
SendWarningPush();
},"two").start();
}
public static synchronized void SendWarningPush(){
System.out.println(Thread.currentThread().getName()+"\t send push msg");
SendWarningSms();
}
public static synchronized void SendWarningSms(){
System.out.println(Thread.currentThread().getName()+"\t send Sms msg");
}
}
看下运行的结果
two send push msg
two send Sms msg
one send push msg
one send Sms msg
接下来看下ReentrantLock 重入锁的演示案例
package com.zz.study.lock;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class TestReentrantLock implements Runnable{
Lock lock=new ReentrantLock();
@Override
public void run() {
getMsg();
}
public void getMsg(){
lock.lock();
lock.lock();
try {
setMsg();
System.out.println(Thread.currentThread().getName()+"\t get()");
}finally {
lock.unlock();
lock.unlock();
}
}
public void setMsg(){
lock.lock();
try {
System.out.println(Thread.currentThread().getName()+"\t set()");
}finally {
lock.unlock();
}
}
public static void main(String[] args) {
Thread t1=new Thread(new TestReentrantLock());
Thread t2=new Thread(new TestReentrantLock());
t1.start();
t2.start();
}
}
运行结果：
Thread-0 set()
Thread-0 get()
Thread-1 set()
Thread-1 get()
Process finished with exit code 0
可以清晰的看到不管是ReentrantLock 还是synchronized 都是可重入的 可以看到同一个线程获取到外层的锁的时候，内层的方法可以自动获取到锁
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