总结写在前面:
RLock rLock = redissonClient.getLock("lbhTestLock");
使用tryLock无参方法时,redisson会自动添加一个定时任务,定时刷新锁的失效时间,如果unlock时失败,则会出现该锁一直不释放的情况。
而当tryLock传释放时间时,则不会添加这个定时任务。
测试如下:
1、tryLock 无参数
@Test
public void testLock() throws Exception {
RLock rLock = redissonClient.getLock("lbhTestLock");
rLock.tryLock();
new Thread(new Runnable() {
@Override
public void run() {
try {
if (rLock.tryLock()) {
System.out.println("成功获取锁");
} else {
System.out.println("未获取锁");
}
Thread.sleep(35000);
if (rLock.tryLock()) {
System.out.println("成功获取锁");
} else {
System.out.println("未获取锁");
}
} catch (Exception e) {
}
}
}).start();
Thread.sleep(40000);
rLock.unlock();
}
输出:
未获取锁
未获取锁
2、tryLock 传释放时间
rLock.tryLock(0,30,TimeUnit.SECONDS);
输出:
未获取锁
成功获取锁
经测试
@Test
public void testLock2() throws Exception {
new Thread(new Runnable() {
@Override
public void run() {
RLock rLock = redissonClient.getLock("lbhTestLock");
rLock.tryLock();
}
}).start();
Thread.sleep(600000);
//rLock.unlock();
}
redis控制台:
test01:0>ttl lbhTestLock
"29"
test01:0>ttl lbhTestLock
"28"
test01:0>ttl lbhTestLock
"27"
test01:0>ttl lbhTestLock
"25"
test01:0>ttl lbhTestLock
"24"
test01:0>ttl lbhTestLock
"22"
test01:0>ttl lbhTestLock
"21"
//过期时间被重置了
test01:0>ttl lbhTestLock
"28"
// 程序关闭后
test01:0>ttl lbhTestLock
"18"
test01:0>ttl lbhTestLock
"15"
test01:0>ttl lbhTestLock
"13"
如果调用了tryLock()缺没有unlock或者unlock时失败,如果程序不关闭,另外一个线程永远不会获取到这个锁,因为redisson会刷新该锁的过期时间, ttl 会一直重置,除非redisson实例关闭/程序关闭,由redis自动清除
源码分析:
调用tryLock() 无参方法时:
@Override
public boolean tryLock() {
return get(tryLockAsync());
}
@Override
public RFuture tryLockAsync() {
return tryLockAsync(Thread.currentThread().getId());
}
public RFuture tryLockAsync(long threadId) {
return tryAcquireOnceAsync(-1, null, threadId);
}
private RFuture tryAcquireOnceAsync(long leaseTime, TimeUnit unit, final long threadId) {
//无参时不走这一步
if (leaseTime != -1) {
return tryLockInnerAsync(leaseTime, unit, threadId, RedisCommands.EVAL_NULL_BOOLEAN);
}
// 尝试获取锁
RFuture ttlRemainingFuture = tryLockInnerAsync(LOCK_EXPIRATION_INTERVAL_SECONDS, TimeUnit.SECONDS, threadId, RedisCommands.EVAL_NULL_BOOLEAN);
// 添加Listener 重点!
ttlRemainingFuture.addListener(new FutureListener() {
@Override
public void operationComplete(Future future) throws Exception {
if (!future.isSuccess()) {
return;
}
Boolean ttlRemaining = future.getNow();
// lock acquired
// 第一次tryLock时一定会成功,即每次新的锁调用tryLock时都会触发
if (ttlRemaining) {
//添加定时任务 定时刷新锁的过期时间
scheduleExpirationRenewal(threadId);
}
}
});
return ttlRemainingFuture;
}
private void scheduleExpirationRenewal(final long threadId) {
if (expirationRenewalMap.containsKey(getEntryName())) {
return;
}
Timeout task = commandExecutor.getConnectionManager().newTimeout(new TimerTask() {
@Override
public void run(Timeout timeout) throws Exception {
// 刷新过期时间
RFuture future = commandExecutor.evalWriteAsync(getName(), LongCodec.INSTANCE, RedisCommands.EVAL_BOOLEAN,
"if (redis.call('hexists', KEYS[1], ARGV[2]) == 1) then " +
//设置key过期时间{防止获取锁后线程挂掉导致死锁}
"redis.call('pexpire', KEYS[1], ARGV[1]); " +
"return 1; " +
"end; " +
"return 0;",
Collections.
添加一个定时任务 每隔过期时间的1/3(默认30s的1/3 10s)刷新锁的过期时间
//lua脚本解析
首先判断这个锁key的map结构中是否存在对应的key8a9649f5-f5b5-48b4-beaa-d0c24855f9ab:anyLock:1,如果存在,就直接调用pexpire命令设置锁key的过期时间,默认30000毫秒。
关于这一原理,可以参考:https://blog.csdn.net/ice24for/article/details/86177152