Java多线程系列--“JUC线程池”03之 线程池原理
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概要
在前面一章"Java多线程系列--“JUC线程池”02之 线程池原理(一)"中介绍了线程池的数据结构，本章会通过分析线程池的源码，对线程池进行说明。内容包括：
- 线程池示例
- 参考代码(基于JDK1.7.0_40)
- 线程池源码分析
- (一) 创建“线程池”
- (二) 添加任务到“线程池”
- (三) 关闭“线程池”
线程池示例
在分析线程池之前，先看一个简单的线程池示例。
import java.util.concurrent.Executors; import java.util.concurrent.ExecutorService; public class ThreadPoolDemo1 { public static void main(String[] args) { // 创建一个可重用固定线程数的线程池 ExecutorService pool = Executors.newFixedThreadPool(2); // 创建实现了Runnable接口对象，Thread对象当然也实现了Runnable接口 Thread ta = new MyThread(); Thread tb = new MyThread(); Thread tc = new MyThread(); Thread td = new MyThread(); Thread te = new MyThread(); // 将线程放入池中进行执行 pool.execute(ta); pool.execute(tb); pool.execute(tc); pool.execute(td); pool.execute(te); // 关闭线程池 pool.shutdown(); } } class MyThread extends Thread { @Override public void run() { System.out.println(Thread.currentThread().getName()+ " is running."); } }
运行结果：
pool-1-thread-1 is running. pool-1-thread-2 is running. pool-1-thread-1 is running. pool-1-thread-2 is running. pool-1-thread-1 is running.
示例中，包括了线程池的创建，将任务添加到线程池中，关闭线程池这3个主要的步骤。稍后，我们会从这3个方面来分析ThreadPoolExecutor。
参考代码(基于JDK1.7.0_40)
Executors完整源码
1 /* 2 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. 3 * 4 * 5 * 6 * 7 * 8 * 9 * 10 * 11 * 12 * 13 * 14 * 15 * 16 * 17 * 18 * 19 * 20 * 21 * 22 * 23 */ 24 25 /* 26 * 27 * 28 * 29 * 30 * 31 * Written by Doug Lea with assistance from members of JCP JSR-166 32 * Expert Group and released to the public domain, as explained at 33 * http://creativecommons.org/publicdomain/zero/1.0/ 34 */ 35 36 package java.util.concurrent; 37 import java.util.*; 38 import java.util.concurrent.atomic.AtomicInteger; 39 import java.security.AccessControlContext; 40 import java.security.AccessController; 41 import java.security.PrivilegedAction; 42 import java.security.PrivilegedExceptionAction; 43 import java.security.PrivilegedActionException; 44 import java.security.AccessControlException; 45 import sun.security.util.SecurityConstants; 46 47 /** 48 * Factory and utility methods for {@link Executor}, {@link 49 * ExecutorService}, {@link ScheduledExecutorService}, {@link 50 * ThreadFactory}, and {@link Callable} classes defined in this 51 * package. This class supports the following kinds of methods: 52 * 53 * <ul> 54 * <li> Methods that create and return an {@link ExecutorService} 55 * set up with commonly useful configuration settings. 56 * <li> Methods that create and return a {@link ScheduledExecutorService} 57 * set up with commonly useful configuration settings. 58 * <li> Methods that create and return a "wrapped" ExecutorService, that 59 * disables reconfiguration by making implementation-specific methods 60 * inaccessible. 61 * <li> Methods that create and return a {@link ThreadFactory} 62 * that sets newly created threads to a known state. 63 * <li> Methods that create and return a {@link Callable} 64 * out of other closure-like forms, so they can be used 65 * in execution methods requiring <tt>Callable</tt>. 66 * </ul> 67 * 68 * @since 1.5 69 * @author Doug Lea 70 */ 71 public class Executors { 72 73 /** 74 * Creates a thread pool that reuses a fixed number of threads 75 * operating off a shared unbounded queue. At any point, at most 76 * <tt>nThreads</tt> threads will be active processing tasks. 77 * If additional tasks are submitted when all threads are active, 78 * they will wait in the queue until a thread is available. 79 * If any thread terminates due to a failure during execution 80 * prior to shutdown, a new one will take its place if needed to 81 * execute subsequent tasks. The threads in the pool will exist 82 * until it is explicitly {@link ExecutorService#shutdown shutdown}. 83 * 84 * @param nThreads the number of threads in the pool 85 * @return the newly created thread pool 86 * @throws IllegalArgumentException if {@code nThreads <= 0} 87 */ 88 public static ExecutorService newFixedThreadPool(int nThreads) { 89 return new ThreadPoolExecutor(nThreads, nThreads, 90 0L, TimeUnit.MILLISECONDS, 91 new LinkedBlockingQueue<Runnable>()); 92 } 93 94 /** 95 * Creates a thread pool that reuses a fixed number of threads 96 * operating off a shared unbounded queue, using the provided 97 * ThreadFactory to create new threads when needed. At any point, 98 * at most <tt>nThreads</tt> threads will be active processing 99 * tasks. If additional tasks are submitted when all threads are 100 * active, they will wait in the queue until a thread is 101 * available. If any thread terminates due to a failure during 102 * execution prior to shutdown, a new one will take its place if 103 * needed to execute subsequent tasks. The threads in the pool will 104 * exist until it is explicitly {@link ExecutorService#shutdown 105 * shutdown}. 106 * 107 * @param nThreads the number of threads in the pool 108 * @param threadFactory the factory to use when creating new threads 109 * @return the newly created thread pool 110 * @throws NullPointerException if threadFactory is null 111 * @throws IllegalArgumentException if {@code nThreads <= 0} 112 */ 113 public static ExecutorService newFixedThreadPool(int nThreads, ThreadFactory threadFactory) { 114 return new ThreadPoolExecutor(nThreads, nThreads, 115 0L, TimeUnit.MILLISECONDS, 116 new LinkedBlockingQueue<Runnable>(), 117 threadFactory); 118 } 119 120 /** 121 * Creates an Executor that uses a single worker thread operating 122 * off an unbounded queue. (Note however that if this single 123 * thread terminates due to a failure during execution prior to 124 * shutdown, a new one will take its place if needed to execute 125 * subsequent tasks.) Tasks are guaranteed to execute 126 * sequentially, and no more than one task will be active at any 127 * given time. Unlike the otherwise equivalent 128 * <tt>newFixedThreadPool(1)</tt> the returned executor is 129 * guaranteed not to be reconfigurable to use additional threads. 130 * 131 * @return the newly created single-threaded Executor 132 */ 133 public static ExecutorService newSingleThreadExecutor() { 134 return new FinalizableDelegatedExecutorService 135 (new ThreadPoolExecutor(1, 1, 136 0L, TimeUnit.MILLISECONDS, 137 new LinkedBlockingQueue<Runnable>())); 138 } 139 140 /** 141 * Creates an Executor that uses a single worker thread operating 142 * off an unbounded queue, and uses the provided ThreadFactory to 143 * create a new thread when needed. Unlike the otherwise 144 * equivalent <tt>newFixedThreadPool(1, threadFactory)</tt> the 145 * returned executor is guaranteed not to be reconfigurable to use 146 * additional threads. 147 * 148 * @param threadFactory the factory to use when creating new 149 * threads 150 * 151 * @return the newly created single-threaded Executor 152 * @throws NullPointerException if threadFactory is null 153 */ 154 public static ExecutorService newSingleThreadExecutor(ThreadFactory threadFactory) { 155 return new FinalizableDelegatedExecutorService 156 (new ThreadPoolExecutor(1, 1, 157 0L, TimeUnit.MILLISECONDS, 158 new LinkedBlockingQueue<Runnable>(), 159 threadFactory)); 160 } 161 162 /** 163 * Creates a thread pool that creates new threads as needed, but 164 * will reuse previously constructed threads when they are 165 * available. These pools will typically improve the performance 166 * of programs that execute many short-lived asynchronous tasks. 167 * Calls to <tt>execute</tt> will reuse previously constructed 168 * threads if available. If no existing thread is available, a new 169 * thread will be created and added to the pool. Threads that have 170 * not been used for sixty seconds are terminated and removed from 171 * the cache. Thus, a pool that remains idle for long enough will 172 * not consume any resources. Note that pools with similar 173 * properties but different details (for example, timeout parameters) 174 * may be created using {@link ThreadPoolExecutor} constructors. 175 * 176 * @return the newly created thread pool 177 */ 178 public static ExecutorService newCachedThreadPool() { 179 return new ThreadPoolExecutor(0, Integer.MAX_VALUE, 180 60L, TimeUnit.SECONDS, 181 new SynchronousQueue<Runnable>()); 182 } 183 184 /** 185 * Creates a thread pool that creates new threads as needed, but 186 * will reuse previously constructed threads when they are 187 * available, and uses the provided 188 * ThreadFactory to create new threads when needed. 189 * @param threadFactory the factory to use when creating new threads 190 * @return the newly created thread pool 191 * @throws NullPointerException if threadFactory is null 192 */ 193 public static ExecutorService newCachedThreadPool(ThreadFactory threadFactory) { 194 return new ThreadPoolExecutor(0, Integer.MAX_VALUE, 195 60L, TimeUnit.SECONDS, 196 new SynchronousQueue<Runnable>(), 197 threadFactory); 198 } 199 200 /** 201 * Creates a single-threaded executor that can schedule commands 202 * to run after a given delay, or to execute periodically. 203 * (Note however that if this single 204 * thread terminates due to a failure during execution prior to 205 * shutdown, a new one will take its place if needed to execute 206 * subsequent tasks.) Tasks are guaranteed to execute 207 * sequentially, and no more than one task will be active at any 208 * given time. Unlike the otherwise equivalent 209 * <tt>newScheduledThreadPool(1)</tt> the returned executor is 210 * guaranteed not to be reconfigurable to use additional threads. 211 * @return the newly created scheduled executor 212 */ 213 public static ScheduledExecutorService newSingleThreadScheduledExecutor() { 214 return new DelegatedScheduledExecutorService 215 (new ScheduledThreadPoolExecutor(1)); 216 } 217 218 /** 219 * Creates a single-threaded executor that can schedule commands 220 * to run after a given delay, or to execute periodically. (Note 221 * however that if this single thread terminates due to a failure 222 * during execution prior to shutdown, a new one will take its 223 * place if needed to execute subsequent tasks.) Tasks are 224 * guaranteed to execute sequentially, and no more than one task 225 * will be active at any given time. Unlike the otherwise 226 * equivalent <tt>newScheduledThreadPool(1, threadFactory)</tt> 227 * the returned executor is guaranteed not to be reconfigurable to 228 * use additional threads. 229 * @param threadFactory the factory to use when creating new 230 * threads 231 * @return a newly created scheduled executor 232 * @throws NullPointerException if threadFactory is null 233 */ 234 public static ScheduledExecutorService newSingleThreadScheduledExecutor(ThreadFactory threadFactory) { 235 return new DelegatedScheduledExecutorService 236 (new ScheduledThreadPoolExecutor(1, threadFactory)); 237 } 238 239 /** 240 * Creates a thread pool that can schedule commands to run after a 241 * given delay, or to execute periodically. 242 * @param corePoolSize the number of threads to keep in the pool, 243 * even if they are idle. 244 * @return a newly created scheduled thread pool 245 * @throws IllegalArgumentException if {@code corePoolSize < 0} 246 */ 247 public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) { 248 return new ScheduledThreadPoolExecutor(corePoolSize); 249 } 250 251 /** 252 * Creates a thread pool that can schedule commands to run after a 253 * given delay, or to execute periodically. 254 * @param corePoolSize the number of threads to keep in the pool, 255 * even if they are idle. 256 * @param threadFactory the factory to use when the executor 257 * creates a new thread. 258 * @return a newly created scheduled thread pool 259 * @throws IllegalArgumentException if {@code corePoolSize < 0} 260 * @throws NullPointerException if threadFactory is null 261 */ 262 public static ScheduledExecutorService newScheduledThreadPool( 263 int corePoolSize, ThreadFactory threadFactory) { 264 return new ScheduledThreadPoolExecutor(corePoolSize, threadFactory); 265 } 266 267 268 /** 269 * Returns an object that delegates all defined {@link 270 * ExecutorService} methods to the given executor, but not any 271 * other methods that might otherwise be accessible using 272 * casts. This provides a way to safely "freeze" configuration and 273 * disallow tuning of a given concrete implementation. 274 * @param executor the underlying implementation 275 * @return an <tt>ExecutorService</tt> instance 276 * @throws NullPointerException if executor null 277 */ 278 public static ExecutorService unconfigurableExecutorService(ExecutorService executor) { 279 if (executor == null) 280 throw new NullPointerException(); 281 return new DelegatedExecutorService(executor); 282 } 283 284 /** 285 * Returns an object that delegates all defined {@link 286 * ScheduledExecutorService} methods to the given executor, but 287 * not any other methods that might otherwise be accessible using 288 * casts. This provides a way to safely "freeze" configuration and 289 * disallow tuning of a given concrete implementation. 290 * @param executor the underlying implementation 291 * @return a <tt>ScheduledExecutorService</tt> instance 292 * @throws NullPointerException if executor null 293 */ 294 public static ScheduledExecutorService unconfigurableScheduledExecutorService(ScheduledExecutorService executor) { 295 if (executor == null) 296 throw new NullPointerException(); 297 return new DelegatedScheduledExecutorService(executor); 298 } 299 300 /** 301 * Returns a default thread factory used to create new threads. 302 * This factory creates all new threads used by an Executor in the 303 * same {@link ThreadGroup}. If there is a {@link 304 * java.lang.SecurityManager}, it uses the group of {@link 305 * System#getSecurityManager}, else the group of the thread 306 * invoking this <tt>defaultThreadFactory</tt> method. Each new 307 * thread is created as a non-daemon thread with priority set to 308 * the smaller of <tt>Thread.NORM_PRIORITY</tt> and the maximum 309 * priority permitted in the thread group. New threads have names 310 * accessible via {@link Thread#getName} of 311 * <em>pool-N-thread-M</em>, where <em>N</em> is the sequence 312 * number of this factory, and <em>M</em> is the sequence number 313 * of the thread created by this factory. 314 * @return a thread factory 315 */ 316 public static ThreadFactory defaultThreadFactory() { 317 return new DefaultThreadFactory(); 318 } 319 320 /** 321 * Returns a thread factory used to create new threads that 322 * have the same permissions as the current thread. 323 * This factory creates threads with the same settings as {@link 324 * Executors#defaultThreadFactory}, additionally setting the 325 * AccessControlContext and contextClassLoader of new threads to 326 * be the same as the thread invoking this 327 * <tt>privilegedThreadFactory</tt> method. A new 328 * <tt>privilegedThreadFactory</tt> can be created within an 329 * {@link AccessController#doPrivileged} action setting the 330 * current thread\'s access control context to create threads with 331 * the selected permission settings holding within that action. 332 * 333 * <p> Note that while tasks running within such threads will have 334 * the same access control and class loader settings as the 335 * current thread, they need not have the same {@link 336 * java.lang.ThreadLocal} or {@link 337 * java.lang.InheritableThreadLocal} values. If necessary, 338 * particular values of thread locals can be set or reset before 339 * any task runs in {@link ThreadPoolExecutor} subclasses using 340 * {@link ThreadPoolExecutor#beforeExecute}. Also, if it is 341 * necessary to initialize worker threads to have the same 342 * InheritableThreadLocal settings as some other designated 343 * thread, you can create a custom ThreadFactory in which that 344 * thread waits for and services requests to create others that 345 * will inherit its values. 346 * 347 * @return a thread factory 348 * @throws AccessControlException if the current access control 349 * context does not have permission to both get and set context 350 * class loader. 351 */ 352 public static ThreadFactory privilegedThreadFactory() { 353 return new PrivilegedThreadFactory(); 354<以上是关于Java多线程系列--“JUC线程池”03之 线程池原理的主要内容，如果未能解决你的问题，请参考以下文章