程序清单 14-12 使用 Lock 来实现信号量

Author: zhangquanli

src/main/java/chapter_14/code_12/SemaphoreOnLock.java

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+package chapter_14.code_12;
+
+import net.jcip.annotations.GuardedBy;
+import net.jcip.annotations.ThreadSafe;
+
+import java.util.concurrent.locks.Condition;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
+
+/**
+ * 程序清单 14-12 使用 Lock 来实现信号量
+ */
+@ThreadSafe
+public class SemaphoreOnLock {
+    private final Lock lock = new ReentrantLock();
+    // 条件谓词：permitsAvailable (permits > 0)
+    private final Condition permitsAvailable = lock.newCondition();
+    @GuardedBy("lock")
+    private int permits;
+
+    SemaphoreOnLock(int initialPermits) {
+        lock.lock();
+        try {
+            permits = initialPermits;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    // 阻塞并直到：permitsAvailable
+    public void acquire() throws InterruptedException {
+        lock.lock();
+        try {
+            while (permits <= 0) {
+                permitsAvailable.await();
+            }
+            --permits;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    public void release() {
+        lock.lock();
+        try {
+            ++permits;
+            permitsAvailable.signal();
+        } finally {
+            lock.unlock();
+        }
+    }
+}