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java/org/apache/catalina/valves/SemaphoreValve.java

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+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.catalina.valves;
+
+import java.io.IOException;
+import java.util.concurrent.Semaphore;
+
+import javax.servlet.ServletException;
+
+import org.apache.catalina.LifecycleException;
+import org.apache.catalina.LifecycleState;
+import org.apache.catalina.connector.Request;
+import org.apache.catalina.connector.Response;
+
+
+/**
+ * <p>Implementation of a Valve that limits concurrency.</p>
+ *
+ * <p>This Valve may be attached to any Container, depending on the granularity
+ * of the concurrency control you wish to perform. Note that internally, some
+ * async requests may require multiple serial requests to complete what - to the
+ * user - appears as a single request.</p>
+ *
+ * @author Remy Maucherat
+ */
+public class SemaphoreValve extends ValveBase {
+
+    //------------------------------------------------------ Constructor
+    public SemaphoreValve() {
+        super(true);
+    }
+
+
+    // ----------------------------------------------------- Instance Variables
+
+    /**
+     * Semaphore.
+     */
+    protected Semaphore semaphore = null;
+
+
+    // ------------------------------------------------------------- Properties
+
+
+    /**
+     * Concurrency level of the semaphore.
+     */
+    protected int concurrency = 10;
+    public int getConcurrency() { return concurrency; }
+    public void setConcurrency(int concurrency) { this.concurrency = concurrency; }
+
+
+    /**
+     * Fairness of the semaphore.
+     */
+    protected boolean fairness = false;
+    public boolean getFairness() { return fairness; }
+    public void setFairness(boolean fairness) { this.fairness = fairness; }
+
+
+    /**
+     * Block until a permit is available.
+     */
+    protected boolean block = true;
+    public boolean getBlock() { return block; }
+    public void setBlock(boolean block) { this.block = block; }
+
+
+    /**
+     * Block interruptibly until a permit is available.
+     */
+    protected boolean interruptible = false;
+    public boolean getInterruptible() { return interruptible; }
+    public void setInterruptible(boolean interruptible) { this.interruptible = interruptible; }
+
+
+    /**
+     * Start this component and implement the requirements
+     * of {@link org.apache.catalina.util.LifecycleBase#startInternal()}.
+     *
+     * @exception LifecycleException if this component detects a fatal error
+     *  that prevents this component from being used
+     */
+    @Override
+    protected synchronized void startInternal() throws LifecycleException {
+
+        semaphore = new Semaphore(concurrency, fairness);
+
+        setState(LifecycleState.STARTING);
+    }
+
+
+    /**
+     * Stop this component and implement the requirements
+     * of {@link org.apache.catalina.util.LifecycleBase#stopInternal()}.
+     *
+     * @exception LifecycleException if this component detects a fatal error
+     *  that prevents this component from being used
+     */
+    @Override
+    protected synchronized void stopInternal() throws LifecycleException {
+
+        setState(LifecycleState.STOPPING);
+
+        semaphore = null;
+    }
+
+
+    // --------------------------------------------------------- Public Methods
+
+    /**
+     * Do concurrency control on the request using the semaphore.
+     *
+     * @param request The servlet request to be processed
+     * @param response The servlet response to be created
+     *
+     * @exception IOException if an input/output error occurs
+     * @exception ServletException if a servlet error occurs
+     */
+    @Override
+    public void invoke(Request request, Response response)
+        throws IOException, ServletException {
+
+        if (controlConcurrency(request, response)) {
+            boolean shouldRelease = true;
+            try {
+                if (block) {
+                    if (interruptible) {
+                        try {
+                            semaphore.acquire();
+                        } catch (InterruptedException e) {
+                            shouldRelease = false;
+                            permitDenied(request, response);
+                            return;
+                        }
+                    } else {
+                        semaphore.acquireUninterruptibly();
+                    }
+                } else {
+                    if (!semaphore.tryAcquire()) {
+                        shouldRelease = false;
+                        permitDenied(request, response);
+                        return;
+                    }
+                }
+                getNext().invoke(request, response);
+            } finally {
+                if (shouldRelease) {
+                    semaphore.release();
+                }
+            }
+        } else {
+            getNext().invoke(request, response);
+        }
+
+    }
+
+
+    /**
+     * Subclass friendly method to add conditions.
+     * @param request The Servlet request
+     * @param response The Servlet response
+     * @return <code>true</code> if the concurrency control should occur
+     *  on this request
+     */
+    public boolean controlConcurrency(Request request, Response response) {
+        return true;
+    }
+
+
+    /**
+     * Subclass friendly method to add error handling when a permit isn't
+     * granted.
+     * @param request The Servlet request
+     * @param response The Servlet response
+     * @throws IOException Error writing output
+     * @throws ServletException Other error
+     */
+    public void permitDenied(Request request, Response response)
+        throws IOException, ServletException {
+        // NO-OP by default
+    }
+
+
+}