1.Semaphore 和 ReentrantLock 的condition有什么不同（acquire()/release()、await()/signal()）？
这里有关于此问题的详尽问答，下面贴下截图。
大意：java.util.concurrent.Semaphore是相对高级的同步机制，一般用来限制并发访问同一资源的线程数量，通过每个线程访问资源之前调用acquire()方法，这样会使permits数量-1.访问结束之后调用release()方法，这样会使permits数量+1. java.util.concurrent.locks.Condition是相对来说低级别的同步机制，可以看作增强版本的java.lang.Object notify() and wait()。
2.Semaphore 和 ReentrantLock不同？
这里有关于该问题的问答。
3.信号量Demo和结果
package com.fqy.mix;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
import java.util.concurrent.TimeUnit;
public class SemaphoreDemo {
public static void main(String[] args) {
SemaphoreDemoUtil.demonstrate();
}
}
class SemaphoreDemoUtil {
public static void demonstrate() {
Long startTime = System.currentTimeMillis();
Connection connection = Connection.getInstance();
ExecutorService executor = Executors.newCachedThreadPool();
for (int i = 0; i < 30; i++) {
executor.submit(new Runnable() {
@Override
public void run() {
try {
connection.doConnect();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
});
}
executor.shutdown();
try {
executor.awaitTermination(1, TimeUnit.MINUTES);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
Long endTime = System.currentTimeMillis();
System.out.println("Total taken time is: " + (endTime - startTime));
}
}
class Connection {
private int count = 0;
private Semaphore semaphore = new Semaphore(10);
private Connection() {
}
private static final Connection INSTANCE = new Connection();
public static Connection getInstance() {
return INSTANCE;
}
private void connect() {
synchronized (this) {
count++;
System.out.println("Current connections is: " + count);
}
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (this) {
count--;
}
}
public void doConnect() throws InterruptedException {
try {
semaphore.acquire();
connect();
} finally {
semaphore.release();
}
}
}
运行结果：
Current connections is: 1
Current connections is: 2
Current connections is: 3
Current connections is: 4
Current connections is: 5
Current connections is: 6
Current connections is: 7
Current connections is: 8
Current connections is: 9
Current connections is: 10
Current connections is: 1
Current connections is: 2
Current connections is: 3
Current connections is: 4
Current connections is: 5
Current connections is: 6
Current connections is: 7
Current connections is: 8
Current connections is: 9
Current connections is: 10
Current connections is: 1
Current connections is: 2
Current connections is: 3
Current connections is: 4
Current connections is: 5
Current connections is: 6
Current connections is: 7
Current connections is: 8
Current connections is: 9
Current connections is: 10
Total taken time is: 3013