1 /*
2 * Copyright 2002-2008 the original author or authors.
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17 package org.springframework.util;
18
19 import java.io.IOException;
20 import java.io.ObjectInputStream;
21 import java.io.Serializable;
22
23 import org.apache.commons.logging.Log;
24 import org.apache.commons.logging.LogFactory;
25
26 /**
27 * Support class for throttling concurrent access to a specific resource.
28 *
29 * <p>Designed for use as a base class, with the subclass invoking
30 * the {@link #beforeAccess()} and {@link #afterAccess()} methods at
31 * appropriate points of its workflow. Note that <code>afterAccess</code>
32 * should usually be called in a finally block!
33 *
34 * <p>The default concurrency limit of this support class is -1
35 * ("unbounded concurrency"). Subclasses may override this default;
36 * check the javadoc of the concrete class that you're using.
37 *
38 * @author Juergen Hoeller
39 * @since 1.2.5
40 * @see #setConcurrencyLimit
41 * @see #beforeAccess()
42 * @see #afterAccess()
43 * @see org.springframework.aop.interceptor.ConcurrencyThrottleInterceptor
44 * @see java.io.Serializable
45 */
46 public abstract class ConcurrencyThrottleSupport implements Serializable {
47
48 /**
49 * Permit any number of concurrent invocations: that is, don't throttle concurrency.
50 */
51 public static final int UNBOUNDED_CONCURRENCY = -1;
52
53 /**
54 * Switch concurrency 'off': that is, don't allow any concurrent invocations.
55 */
56 public static final int NO_CONCURRENCY = 0;
57
58
59 /** Transient to optimize serialization */
60 protected transient Log logger = LogFactory.getLog(getClass());
61
62 private transient Object monitor = new Object();
63
64 private int concurrencyLimit = UNBOUNDED_CONCURRENCY;
65
66 private int concurrencyCount = 0;
67
68
69 /**
70 * Set the maximum number of concurrent access attempts allowed.
71 * -1 indicates unbounded concurrency.
72 * <p>In principle, this limit can be changed at runtime,
73 * although it is generally designed as a config time setting.
74 * <p>NOTE: Do not switch between -1 and any concrete limit at runtime,
75 * as this will lead to inconsistent concurrency counts: A limit
76 * of -1 effectively turns off concurrency counting completely.
77 */
78 public void setConcurrencyLimit(int concurrencyLimit) {
79 this.concurrencyLimit = concurrencyLimit;
80 }
81
82 /**
83 * Return the maximum number of concurrent access attempts allowed.
84 */
85 public int getConcurrencyLimit() {
86 return this.concurrencyLimit;
87 }
88
89 /**
90 * Return whether this throttle is currently active.
91 * @return <code>true</code> if the concurrency limit for this instance is active
92 * @see #getConcurrencyLimit()
93 */
94 public boolean isThrottleActive() {
95 return (this.concurrencyLimit > 0);
96 }
97
98
99 /**
100 * To be invoked before the main execution logic of concrete subclasses.
101 * <p>This implementation applies the concurrency throttle.
102 * @see #afterAccess()
103 */
104 protected void beforeAccess() {
105 if (this.concurrencyLimit == NO_CONCURRENCY) {
106 throw new IllegalStateException(
107 "Currently no invocations allowed - concurrency limit set to NO_CONCURRENCY");
108 }
109 if (this.concurrencyLimit > 0) {
110 boolean debug = logger.isDebugEnabled();
111 synchronized (this.monitor) {
112 boolean interrupted = false;
113 while (this.concurrencyCount >= this.concurrencyLimit) {
114 if (interrupted) {
115 throw new IllegalStateException("Thread was interrupted while waiting for invocation access, " +
116 "but concurrency limit still does not allow for entering");
117 }
118 if (debug) {
119 logger.debug("Concurrency count " + this.concurrencyCount +
120 " has reached limit " + this.concurrencyLimit + " - blocking");
121 }
122 try {
123 this.monitor.wait();
124 }
125 catch (InterruptedException ex) {
126 // Re-interrupt current thread, to allow other threads to react.
127 Thread.currentThread().interrupt();
128 interrupted = true;
129 }
130 }
131 if (debug) {
132 logger.debug("Entering throttle at concurrency count " + this.concurrencyCount);
133 }
134 this.concurrencyCount++;
135 }
136 }
137 }
138
139 /**
140 * To be invoked after the main execution logic of concrete subclasses.
141 * @see #beforeAccess()
142 */
143 protected void afterAccess() {
144 if (this.concurrencyLimit >= 0) {
145 synchronized (this.monitor) {
146 this.concurrencyCount--;
147 if (logger.isDebugEnabled()) {
148 logger.debug("Returning from throttle at concurrency count " + this.concurrencyCount);
149 }
150 this.monitor.notify();
151 }
152 }
153 }
154
155
156 //---------------------------------------------------------------------
157 // Serialization support
158 //---------------------------------------------------------------------
159
160 private void readObject(ObjectInputStream ois) throws IOException, ClassNotFoundException {
161 // Rely on default serialization, just initialize state after deserialization.
162 ois.defaultReadObject();
163
164 // Initialize transient fields.
165 this.logger = LogFactory.getLog(getClass());
166 this.monitor = new Object();
167 }
168
169 }