使用synchronized实现一个Lock代码详解
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2023-12-14 12:54:16
刚看到这个题目的时候无从下手，因为觉得synchronized和lock在加锁的方式上有很大不同，比如，看看正常情况下synchronized时如何加锁的。
方式一：...
刚看到这个题目的时候无从下手，因为觉得synchronized和lock在加锁的方式上有很大不同，比如，看看正常情况下synchronized时如何加锁的。
方式一：
public synchronized void a(){
//todo
}
方式二：
public void b(){
synchronized(this){
//todo
}
}
从这两种方式来看，锁都是加在｛｝之间的，我们再来看看lock是如何做的呢:
public void c() {
lock.lock();
try {
// todo
} finally {
lock.unlock();
}
}
这种方式的锁是加在lock()和unlock()之间的，所以要想实现一个lock功能，就要想怎么实现这样两个方法，lock()和unlock()方法，先定义一个框架如下所示：
public void lock(){
}
public void unlock(){
}
然后要想怎么用synchronized去实现这两个方法。
现在其实只是稍微清楚了一点思路，但是还不知道怎么去填充这两个方法，这是后再来分析一下lock的加锁有什么特点，再来看看这段代码：
public void c() {
lock.lock();
//when current thread get the lock, other thread has to wait
try {
//current thread get in the lock, other thread can not get in
// todo
}
finally {
lock.unlock();
//current thread release the lock
}
}
这段代码我只是加了一点注释，别的什么都没有做，是不是帮助理解这段代码，看看出现频率最高的词是什么，是currentthread，那么我们去填充lock()和unlock()方法的时候是不是注意要抓住currentthread这个关键字就可以找到解决方案呢？答案是肯定的。
接着分析，使用synchronized的时候如何让线程等待呢？是用wait()方法。怎么让线程唤醒呢，是用notify()方法。那么就要在lock()方法中使用wait()方法，在unlock()方法中使用notify()方法。那么我们在使用wait()和notify()的时候是有一个条件的，想想我们应该使用什么作为条件呢？
我们应该使用当前锁是否被占用作为判断条件，如果锁被占用，currentthread等待，想想我们在使用synchronized的时候是不是一直使用的这个条件，答案也是肯定的。
再来分析一下什么时候释放锁，使用什么作为条件，想想如果线程a拿到了锁，线程b能释放吗？当然不能，如果b能释放就违反了原则，当然不能。肯定是a线程的锁只能a来释放。所以判断条件就是判断持有锁的线程是不是currentthread，如果是的话，可以释放，不是的话当然不能。
现在来看看完整的代码：
package test.lock;
import java.util.random;
import java.util.concurrent.executorservice;
import java.util.concurrent.executors;
import java.util.concurrent.threadfactory;
public class naivelock {
private static final long none = -1;
private long owner = none;
private boolean islooked() {
return owner != none;
}
public synchronized void lock() {
long currentthreadid = thread.currentthread().getid();
if (owner == currentthreadid) {
throw new illegalstateexception("lock has been acquired by current thread");
}
while (this.islooked()) {
system.out.println(string.format("thread %s is waitting lock", currentthreadid));
try {
wait();
}
catch (interruptedexception e) {
e.printstacktrace();
}
}
owner = currentthreadid;
system.out.println(string.format("lock is acquired by thread %s", owner));
}
public synchronized void unlock() {
if (!this.islooked() || owner != thread.currentthread().getid()) {
throw new illegalstateexception("only lock owner can unlock the lock");
}
system.out.println(string.format("thread %s is unlocking", owner));
system.out.println();
owner = none;
notify();
}
public static void main(string[] args) {
final naivelock lock = new naivelock();
executorservice executor = executors.newfixedthreadpool(20, new threadfactory() {
private threadgroup group = new threadgroup("test thread group");
{
group.setdaemon(true);
}
@override
public thread newthread(runnable r) {
return new thread(group, r);
}
}
);
for (int i = 0; i < 20; i++) {
executor.submit(new runnable() {
@override
public void run() {
lock.lock();
system.out.println(string.format("thread %s is running...", thread.currentthread().getid()));
try {
thread.sleep(new random().nextint(1000));
}
catch (interruptedexception e) {
e.printstacktrace();
}
lock.unlock();
}
}
);
}
}
}
运行一下看看结果：
lock is acquired by thread 8 thread 8 is running... thread 27 is waitting lock thread 26 is waitting lock thread 25 is waitting lock thread 24 is waitting lock thread 23 is waitting lock thread 22 is waitting lock thread 21 is waitting lock thread 20 is waitting lock thread 19 is waitting lock thread 18 is waitting lock thread 17 is waitting lock thread 16 is waitting lock thread 15 is waitting lock thread 14 is waitting lock thread 13 is waitting lock thread 12 is waitting lock thread 11 is waitting lock thread 10 is waitting lock thread 9 is waitting lock thread 8 is unlocking lock is acquired by thread 27 thread 27 is running... thread 27 is unlocking lock is acquired by thread 26 thread 26 is running... thread 26 is unlocking lock is acquired by thread 25 thread 25 is running... thread 25 is unlocking lock is acquired by thread 24 thread 24 is running... thread 24 is unlocking lock is acquired by thread 23 thread 23 is running... thread 23 is unlocking lock is acquired by thread 22 thread 22 is running... thread 22 is unlocking lock is acquired by thread 21 thread 21 is running... thread 21 is unlocking lock is acquired by thread 20 thread 20 is running... thread 20 is unlocking lock is acquired by thread 19 thread 19 is running... thread 19 is unlocking lock is acquired by thread 18 thread 18 is running... thread 18 is unlocking lock is acquired by thread 17 thread 17 is running... thread 17 is unlocking lock is acquired by thread 16 thread 16 is running... thread 16 is unlocking lock is acquired by thread 15 thread 15 is running... thread 15 is unlocking lock is acquired by thread 14 thread 14 is running... thread 14 is unlocking lock is acquired by thread 13 thread 13 is running... thread 13 is unlocking lock is acquired by thread 12 thread 12 is running... thread 12 is unlocking lock is acquired by thread 11 thread 11 is running... thread 11 is unlocking lock is acquired by thread 10 thread 10 is running... thread 10 is unlocking lock is acquired by thread 9 thread 9 is running... thread 9 is unlocking
如果把for循环改成30次，再看一下结果：
lock is acquired by thread 8 thread 8 is running... thread 27 is waitting lock thread 26 is waitting lock thread 25 is waitting lock thread 24 is waitting lock thread 23 is waitting lock thread 22 is waitting lock thread 21 is waitting lock thread 20 is waitting lock thread 19 is waitting lock thread 18 is waitting lock thread 17 is waitting lock thread 16 is waitting lock thread 15 is waitting lock thread 14 is waitting lock thread 13 is waitting lock thread 12 is waitting lock thread 11 is waitting lock thread 10 is waitting lock thread 9 is waitting lock thread 8 is unlocking lock is acquired by thread 27 thread 27 is running... thread 8 is waitting lock thread 27 is unlocking lock is acquired by thread 27 thread 27 is running... thread 26 is waitting lock thread 27 is unlocking lock is acquired by thread 27 thread 27 is running... thread 25 is waitting lock thread 27 is unlocking lock is acquired by thread 24 thread 24 is running... thread 27 is waitting lock thread 24 is unlocking lock is acquired by thread 23 thread 23 is running... thread 24 is waitting lock thread 23 is unlocking lock is acquired by thread 22 thread 22 is running... thread 23 is waitting lock thread 22 is unlocking lock is acquired by thread 22 thread 22 is running... thread 21 is waitting lock thread 22 is unlocking lock is acquired by thread 22 thread 22 is running... thread 20 is waitting lock thread 22 is unlocking lock is acquired by thread 22 thread 22 is running... thread 19 is waitting lock thread 22 is unlocking lock is acquired by thread 22 thread 22 is running... thread 18 is waitting lock thread 22 is unlocking lock is acquired by thread 17 thread 17 is running... thread 17 is unlocking lock is acquired by thread 16 thread 16 is running... thread 16 is unlocking lock is acquired by thread 15 thread 15 is running... thread 15 is unlocking lock is acquired by thread 14 thread 14 is running... thread 14 is unlocking lock is acquired by thread 13 thread 13 is running... thread 13 is unlocking lock is acquired by thread 12 thread 12 is running... thread 12 is unlocking lock is acquired by thread 11 thread 11 is running... thread 11 is unlocking lock is acquired by thread 10 thread 10 is running... thread 10 is unlocking lock is acquired by thread 9 thread 9 is running... thread 9 is unlocking lock is acquired by thread 8 thread 8 is running... thread 8 is unlocking lock is acquired by thread 26 thread 26 is running... thread 26 is unlocking lock is acquired by thread 25 thread 25 is running... thread 25 is unlocking lock is acquired by thread 27 thread 27 is running... thread 27 is unlocking lock is acquired by thread 24 thread 24 is running... thread 24 is unlocking lock is acquired by thread 23 thread 23 is running... thread 23 is unlocking lock is acquired by thread 21 thread 21 is running... thread 21 is unlocking lock is acquired by thread 20 thread 20 is running... thread 20 is unlocking lock is acquired by thread 19 thread 19 is running... thread 19 is unlocking lock is acquired by thread 18 thread 18 is running... thread 18 is unlocking
总结
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