自定義同步鎖的實現
本文出自《java 併發編程的藝術》
在多線程訪問資源的時候分爲獨佔式與共享式,本例子是一個基於AbstractQueuedSynchronizer的自定義同步鎖;
關於AbstractQueuedSynchronizer的實現原理,其實就是內部維護了一個雙向隊列,在本例子中,如果線程未獲取到鎖的時候會發生自旋。
一下是自定義同步鎖的實現:
package com.kangxin.doctor.connectdemo;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
public class TwinsLock implements Lock {
private final Sync sync = new Sync(2);
@Override
public void lock() {
sync.acquireShared(1);
}
@Override
public void lockInterruptibly() throws InterruptedException {
}
@Override
public boolean tryLock() {
return false;
}
@Override
public boolean tryLock(long time, TimeUnit unit) throws InterruptedException {
return false;
}
@Override
public void unlock() {
sync.releaseShared(1);
}
@Override
public Condition newCondition() {
return null;
}
private static final class Sync extends AbstractQueuedSynchronizer{
Sync(int count) {
if (count <=0){
throw new IllegalArgumentException("count must large than zero.");
}
setState(count);
}
@Override
protected int tryAcquireShared(int reduceCount) {
for (;;){
int current = getState();
int newCount = current - reduceCount;
if (newCount < 0 || compareAndSetState(current, newCount)){
return newCount;
}
}
}
@Override
protected boolean tryReleaseShared(int returnCount) {
for (;;){
int current = getState();
int newCount = current + returnCount;
if (compareAndSetState(current, newCount)){
return true;
}
}
}
}
}
可以看到在這個類中有一個自定義的內部類的實現,該Sync實現了AbstractQueuedSynchronizer的tryAcquireShared方法,由此可知線程的訪問模式是共享式,只不過要根據構造傳入的count來控制共享的線程數量,假設傳入的是2的話,那麼只允許兩個線程共享資源,其他的處於阻塞;
下面通過一個例子來說明一下,這個例子創建來10個線程,只允許兩個線程去共享資源,當前線程處理完操作之後釋放鎖, 好讓其他阻塞的線程來訪問資源。
package com.kangxin.doctor.connectdemo;
import java.util.concurrent.locks.Lock;
public class ShareSyncTest {
public static void main(String[] args) {
final Lock lock = new TwinsLock();
class Worker extends Thread{
@Override
public void run() {
while (true){
lock.lock();
try {
SleepUtil.second(1);
System.out.println(Thread.currentThread().getName());
SleepUtil.second(1);
}finally {
lock.unlock();
}
}
}
}
for (int i=0; i < 10; i++){
Worker w= new Worker();
w.setDaemon(true);
w.start();
}
for (int i=0; i < 10; i++){
SleepUtil.second(1);
System.out.println();
}
}
}
從代碼這可以看到在線程裏面我們用來一個循環類處理,開始的時候獲取鎖然後睡眠來1秒之後打印來一下線程名字然後再睡眠一秒之後釋放了鎖;該示例運行起來之後會發現每隔2秒會同時打印2個線程信息。
效果如下
Thread-0
Thread-1
Thread-1
Thread-0
Thread-0
Thread-1
Thread-0
Thread-1
Thread-0
Thread-1