老婆保佑，代码无BUG
前言
2018 准备换工作，也得好好准备面试，PS，小伙伴需要招人不，求推荐
面试题： 说说你对JAVA 线程池的理解
目录
- 一：线程池的优点
- 二：线程种类
- 三：使用介绍
- FixedThreadPool (定长线程池)
- CacheThreadPool (可缓存线程池)
- SingleThreadExecutor (单线程化线程池)
- ScheduledThreadPool (定时线程池)
- 四：源码分析
- 五：ThreadPoolExecutor参数
- 参考
一：线程池的优点
- 减少在创建和销毁线程上所花的时间以及系统资源的开销。
- 有效控制线程并发，避免大量线程之间因抢占资源导致阻塞现象
二：线程种类
- FixedThreadPool (定长线程池)
- SingleThreadExecutor (单线程化线程池)
- CachedThreadPool (可缓存线程池)
- ScheduledThreadPool (定时线程池)
三：使用介绍
1. FixedThreadPool
ExecutorService fixedThreadPool = Executors.newFixedThreadPool(3);
特点：
- 这是一种数量固定的线程池,当线程处于空闲的时候,并不会被回收,除非线程池被关闭.
- 当所有的线程都处于活动状态时,新任务都会处于等待状态,直到有线程空闲出来.
- 由于FixedThreadPool中只有核心线程并且这些核心线程不会被回收,这意味着它能够更加快速地响应外界的请求.
eg:
private void init_TestThread() {
ExecutorService fixedThreadPool = Executors.newFixedThreadPool(3);
for (int i = 0; i < 10; i++) {
final int finalI = i;
Runnable runnable = new Runnable() {
@Override
public void run() {
SystemClock.sleep(3000);
Log.d("TAG", "run: " + finalI + " thread: "+Thread.currentThread().getName());
}
};
fixedThreadPool.execute(runnable);
}
}
log:
02-11 02:52:42.609 4305-4464/com.cpsc.rxjavademo D/TAG: run: 1 thread: pool-3-thread-2
02-11 02:52:42.622 4305-4463/com.cpsc.rxjavademo D/TAG: run: 0 thread: pool-3-thread-1
02-11 02:52:42.631 4305-4465/com.cpsc.rxjavademo D/TAG: run: 2 thread: pool-3-thread-3
02-11 02:52:45.612 4305-4464/com.cpsc.rxjavademo D/TAG: run: 3 thread: pool-3-thread-2
02-11 02:52:45.625 4305-4463/com.cpsc.rxjavademo D/TAG: run: 4 thread: pool-3-thread-1
02-11 02:52:45.634 4305-4465/com.cpsc.rxjavademo D/TAG: run: 5 thread: pool-3-thread-3
02-11 02:52:48.614 4305-4464/com.cpsc.rxjavademo D/TAG: run: 6 thread: pool-3-thread-2
02-11 02:52:48.628 4305-4463/com.cpsc.rxjavademo D/TAG: run: 7 thread: pool-3-thread-1
02-11 02:52:48.636 4305-4465/com.cpsc.rxjavademo D/TAG: run: 8 thread: pool-3-thread-3
02-11 02:52:51.617 4305-4464/com.cpsc.rxjavademo D/TAG: run: 9 thread: pool-3-thread-2
2. CacheThreadPool
ExecutorService cachedThreadPool = Executors.newCachedThreadPool();
特点：
-
它是一种线程数量不定的线程池,它只有非核心线程,并且其最大线程数为Integer.MAX_VALUE(也就相当于线程池的线程数量可以无限大).
-
当线程池中所有线程都处于活动的状态时,线程池会创建新的线程来处理新任务,否则就会复用空闲线程来处理.
eg:
private void init_TestThread() {
ExecutorService fixedThreadPool = Executors.newCachedThreadPool();
for (int i = 0; i < 10; i++) {
final int finalI = i;
Runnable runnable = new Runnable() {
@Override
public void run() {
SystemClock.sleep(3000);
Log.d("TAG", "run: " + finalI + " thread: "+Thread.currentThread().getName());
}
};
fixedThreadPool.execute(runnable);
}
}
log:
02-11 02:54:16.162 4305-4510/com.cpsc.rxjavademo D/TAG: run: 0 thread: pool-4-thread-1
02-11 02:54:16.164 4305-4511/com.cpsc.rxjavademo D/TAG: run: 1 thread: pool-4-thread-2
02-11 02:54:16.169 4305-4512/com.cpsc.rxjavademo D/TAG: run: 2 thread: pool-4-thread-3
02-11 02:54:16.170 4305-4513/com.cpsc.rxjavademo D/TAG: run: 3 thread: pool-4-thread-4
02-11 02:54:16.172 4305-4514/com.cpsc.rxjavademo D/TAG: run: 4 thread: pool-4-thread-5
02-11 02:54:16.173 4305-4515/com.cpsc.rxjavademo D/TAG: run: 5 thread: pool-4-thread-6
02-11 02:54:16.176 4305-4516/com.cpsc.rxjavademo D/TAG: run: 6 thread: pool-4-thread-7
02-11 02:54:16.180 4305-4517/com.cpsc.rxjavademo D/TAG: run: 7 thread: pool-4-thread-8
02-11 02:54:16.182 4305-4518/com.cpsc.rxjavademo D/TAG: run: 8 thread: pool-4-thread-9
02-11 02:54:16.184 4305-4520/com.cpsc.rxjavademo D/TAG: run: 9 thread: pool-4-thread-10
3. SingleThreadExecutor
ExecutorService singleThreadExecutor = Executors.newSingleThreadExecutor();
特点
- 这类线程池内部只有一个核心线程,它确保所有的任务都在同一个线程中按顺序执行.
eg:
private void init_TestThread() {
ExecutorService fixedThreadPool = Executors.newSingleThreadExecutor();
for (int i = 0; i < 10; i++) {
final int finalI = i;
Runnable runnable = new Runnable() {
@Override
public void run() {
SystemClock.sleep(3000);
Log.d("TAG", "run: " + finalI + " thread: "+Thread.currentThread().getName());
}
};
fixedThreadPool.execute(runnable);
}
}
log:
02-11 02:55:26.466 4305-4557/com.cpsc.rxjavademo D/TAG: run: 0 thread: pool-5-thread-1
02-11 02:55:29.466 4305-4557/com.cpsc.rxjavademo D/TAG: run: 1 thread: pool-5-thread-1
02-11 02:55:32.472 4305-4557/com.cpsc.rxjavademo D/TAG: run: 2 thread: pool-5-thread-1
02-11 02:55:35.473 4305-4557/com.cpsc.rxjavademo D/TAG: run: 3 thread: pool-5-thread-1
02-11 02:55:38.475 4305-4557/com.cpsc.rxjavademo D/TAG: run: 4 thread: pool-5-thread-1
02-11 02:55:41.476 4305-4557/com.cpsc.rxjavademo D/TAG: run: 5 thread: pool-5-thread-1
02-11 02:55:44.478 4305-4557/com.cpsc.rxjavademo D/TAG: run: 6 thread: pool-5-thread-1
02-11 02:55:47.479 4305-4557/com.cpsc.rxjavademo D/TAG: run: 7 thread: pool-5-thread-1
02-11 02:55:50.480 4305-4557/com.cpsc.rxjavademo D/TAG: run: 8 thread: pool-5-thread-1
02-11 02:55:53.481 4305-4557/com.cpsc.rxjavademo D/TAG: run: 9 thread: pool-5-thread-1
4. ScheduledThreadPool
ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(3);
特点
- 它的核心线程数量是固定的,而非核心线程数是没有限制的,并且当非核心线程闲置时会被立即回收.
eg:
private void init_TestThread() {
ExecutorService fixedThreadPool = Executors.newScheduledThreadPool(3);
for (int i = 0; i < 10; i++) {
final int finalI = i;
Runnable runnable = new Runnable() {
@Override
public void run() {
SystemClock.sleep(3000);
Log.d("TAG", "run: " + finalI + " thread: "+Thread.currentThread().getName());
}
};
fixedThreadPool.execute(runnable);
}
}
log:
02-11 03:41:32.230 6981-7045/com.cpsc.rxjavademo D/TAG: run: 0 thread: pool-1-thread-1
02-11 03:41:32.236 6981-7046/com.cpsc.rxjavademo D/TAG: run: 1 thread: pool-1-thread-2
02-11 03:41:32.239 6981-7047/com.cpsc.rxjavademo D/TAG: run: 2 thread: pool-1-thread-3
02-11 03:41:35.233 6981-7045/com.cpsc.rxjavademo D/TAG: run: 3 thread: pool-1-thread-1
02-11 03:41:35.238 6981-7046/com.cpsc.rxjavademo D/TAG: run: 4 thread: pool-1-thread-2
02-11 03:41:35.240 6981-7047/com.cpsc.rxjavademo D/TAG: run: 5 thread: pool-1-thread-3
02-11 03:41:38.234 6981-7045/com.cpsc.rxjavademo D/TAG: run: 6 thread: pool-1-thread-1
02-11 03:41:38.240 6981-7046/com.cpsc.rxjavademo D/TAG: run: 7 thread: pool-1-thread-2
02-11 03:41:38.242 6981-7047/com.cpsc.rxjavademo D/TAG: run: 8 thread: pool-1-thread-3
02-11 03:41:41.237 6981-7045/com.cpsc.rxjavademo D/TAG: run: 9 thread: pool-1-thread-1
四：源码分析
- FixedThreadPool
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
- CacheThreadPool
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}
- SingleThreadExecutor
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>()));
}
- ScheduledThreadPool
public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
return new ScheduledThreadPoolExecutor(corePoolSize);
}
public ScheduledThreadPoolExecutor(int corePoolSize) {
super(corePoolSize, Integer.MAX_VALUE,
DEFAULT_KEEPALIVE_MILLIS, MILLISECONDS,
new DelayedWorkQueue());
}
五：ThreadPoolExecutor参数
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue) {}
参数说明
| 参数 | 说明 |
|---|---|
| corePoolSize | 线程池中核心线程的数量 |
| maximumPoolSize | 线程池中最大线程数量 |
| keepAliveTime | 非核心线程的超时时长，当系统中非核心线程闲置时间超过keepAliveTime之后，则会被回收。如果ThreadPoolExecutor的allowCoreThreadTimeOut属性设置为true，则该参数也表示核心线程的超时时长 |
| unit | 用于指定keepAliveTime参数单位，有纳秒、微秒、毫秒、秒、分、时、天等 |
| workQueue | 线程池中的任务队列，该队列主要用来存储已经被提交但是尚未执行的任务。存储在这里的任务是由ThreadPoolExecutor的execute方法提交来的。 |
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