Executors生成常用的几种线程池执行者
- 可缓存线程池
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue());
}
public static ExecutorService newCachedThreadPool(ThreadFactory threadFactory) {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue(),
threadFactory);
}
- 定长的线程池
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue());
}
public static ExecutorService newFixedThreadPool(int nThreads, ThreadFactory threadFactory) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue(),
threadFactory);
}
- 定时线程池
public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
return new ScheduledThreadPoolExecutor(corePoolSize);
}
public static ScheduledExecutorService newScheduledThreadPool(
int corePoolSize, ThreadFactory threadFactory) {
return new ScheduledThreadPoolExecutor(corePoolSize, threadFactory);
}
ScheduledThreadPoolExecutor是ThreadPoolExecutor的子类。最后还是调用ThreadPoolExecutor
public ScheduledThreadPoolExecutor(int corePoolSize) {
super(corePoolSize, Integer.MAX_VALUE,
DEFAULT_KEEPALIVE_MILLIS, MILLISECONDS,
new DelayedWorkQueue());
}
public ScheduledThreadPoolExecutor(int corePoolSize,
ThreadFactory threadFactory) {
super(corePoolSize, Integer.MAX_VALUE,
DEFAULT_KEEPALIVE_MILLIS, MILLISECONDS,
new DelayedWorkQueue(), threadFactory);
}
- 单列线程池
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue()));
}
public static ExecutorService newSingleThreadExecutor(ThreadFactory threadFactory) {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue(),
threadFactory));
}
ThreadPoolExecutor参数的解释
/**
*corePoolSize 核心线程数 实际运行的线程数
*maximumPoolSize 最多可创建的线程
*keepAliveTime 非核心线程闲置线程最大存活时间
*TimeUnit keepAliveTime 的时间单位
*threadFactory 线程工厂，好处就是允许应用程序使用特殊的线程子类，设置属性等等
*RejectedExecutionHandler 在方法execute()中提交的新任务将被拒绝后处理方式
* Creates a new {@code ThreadPoolExecutor} with the given initial
* parameters.
*
* @param corePoolSize the number of threads to keep in the pool, even
* if they are idle, unless {@code allowCoreThreadTimeOut} is set
* @param maximumPoolSize the maximum number of threads to allow in the
* pool
* @param keepAliveTime when the number of threads is greater than
* the core, this is the maximum time that excess idle threads
* will wait for new tasks before terminating.
* @param unit the time unit for the {@code keepAliveTime} argument
* @param workQueue the queue to use for holding tasks before they are
* executed. This queue will hold only the {@code Runnable}
* tasks submitted by the {@code execute} method.
* @param threadFactory the factory to use when the executor
* creates a new thread
* @param handler the handler to use when execution is blocked
* because the thread bounds and queue capacities are reached
* @throws IllegalArgumentException if one of the following holds:
* {@code corePoolSize < 0}
* {@code keepAliveTime < 0}
* {@code maximumPoolSize <= 0}
* {@code maximumPoolSize < corePoolSize}
* @throws NullPointerException if {@code workQueue}
* or {@code threadFactory} or {@code handler} is null
*/
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler) {
if (corePoolSize < 0 ||
maximumPoolSize <= 0 ||
maximumPoolSize < corePoolSize ||
keepAliveTime < 0)
throw new IllegalArgumentException();
if (workQueue == null || threadFactory == null || handler == null)
throw new NullPointerException();
this.corePoolSize = corePoolSize;
this.maximumPoolSize = maximumPoolSize;
this.workQueue = workQueue;
this.keepAliveTime = unit.toNanos(keepAliveTime);
this.threadFactory = threadFactory;
this.handler = handler;
}
举例使用：
Fresco线程池使用情况
//ThreadFactory 的用法
public class PriorityThreadFactory implements ThreadFactory {
private final int mThreadPriority;
/**
* Creates a new PriorityThreadFactory with a given priority.
*
* This value should be set to a value compatible with
* {@link android.os.Process#setThreadPriority}, not {@link Thread#setPriority}.
*
*/
public PriorityThreadFactory(int threadPriority) {
mThreadPriority = threadPriority;
}
@Override
public Thread newThread(final Runnable runnable) {
Runnable wrapperRunnable = new Runnable() {
@Override
public void run() {
try {
Process.setThreadPriority(mThreadPriority);
} catch (Throwable t) {
// just to be safe
}
runnable.run();
}
};
return new Thread(wrapperRunnable);
}
}
在android 核心线程数值设置一般通过如下方式获取
public static final int DEFAULT_MAX_NUM_THREADS = Runtime.getRuntime().availableProcessors();
public class DefaultExecutorSupplier implements ExecutorSupplier {
// Allows for simultaneous reads and writes.
private static final int NUM_IO_BOUND_THREADS = 2;
private static final int NUM_LIGHTWEIGHT_BACKGROUND_THREADS = 1;
private final Executor mIoBoundExecutor;
private final Executor mDecodeExecutor;
private final Executor mBackgroundExecutor;
private final Executor mLightWeightBackgroundExecutor;
public DefaultExecutorSupplier(int numCpuBoundThreads) {
ThreadFactory backgroundPriorityThreadFactory =
new PriorityThreadFactory(Process.THREAD_PRIORITY_BACKGROUND);
mIoBoundExecutor = Executors.newFixedThreadPool(NUM_IO_BOUND_THREADS);
mDecodeExecutor = Executors.newFixedThreadPool(
numCpuBoundThreads,
backgroundPriorityThreadFactory);
mBackgroundExecutor = Executors.newFixedThreadPool(
numCpuBoundThreads,
backgroundPriorityThreadFactory);
mLightWeightBackgroundExecutor = Executors.newFixedThreadPool(
NUM_LIGHTWEIGHT_BACKGROUND_THREADS,
backgroundPriorityThreadFactory);
}
@Override
public Executor forLocalStorageRead() {
return mIoBoundExecutor;
}
@Override
public Executor forLocalStorageWrite() {
return mIoBoundExecutor;
}
@Override
public Executor forDecode() {
return mDecodeExecutor;
}
@Override
public Executor forBackgroundTasks() {
return mBackgroundExecutor;
}
@Override
public Executor forLightweightBackgroundTasks() {
return mLightWeightBackgroundExecutor;
}
}
看下bolts给android设置参数 构造Executor
/**
* This was created because the helper methods in {@link java.util.concurrent.Executors} do not work
* as people would normally expect.
*
* Normally, you would think that a cached thread pool would create new threads when necessary,
* queue them when the pool is full, and kill threads when they've been inactive for a certain
* period of time. This is not how {@link java.util.concurrent.Executors#newCachedThreadPool()}
* works.
*
* Instead, {@link java.util.concurrent.Executors#newCachedThreadPool()} executes all tasks on
* a new or cached thread immediately because corePoolSize is 0, SynchronousQueue is a queue with
* size 0 and maxPoolSize is Integer.MAX_VALUE. This is dangerous because it can create an unchecked
* amount of threads.
*/
/* package */ final class AndroidExecutors {
private static final AndroidExecutors INSTANCE = new AndroidExecutors();
private final Executor uiThread;
private AndroidExecutors() {
uiThread = new UIThreadExecutor();
}
/**
* Nexus 5: Quad-Core
* Moto X: Dual-Core
*
* AsyncTask:
* CORE_POOL_SIZE = CPU_COUNT + 1
* MAX_POOL_SIZE = CPU_COUNT * 2 + 1
*
* https://github.com/android/platform_frameworks_base/commit/719c44e03b97e850a46136ba336d729f5fbd1f47
*/
private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
/* package */ static final int CORE_POOL_SIZE = CPU_COUNT + 1;
/* package */ static final int MAX_POOL_SIZE = CPU_COUNT * 2 + 1;
/* package */ static final long KEEP_ALIVE_TIME = 1L;
/**
* Creates a proper Cached Thread Pool. Tasks will reuse cached threads if available
* or create new threads until the core pool is full. tasks will then be queued. If an
* task cannot be queued, a new thread will be created unless this would exceed max pool
* size, then the task will be rejected. Threads will time out after 1 second.
*
* Core thread timeout is only available on android-9+.
*
* @return the newly created thread pool
*/
public static ExecutorService newCachedThreadPool() {
ThreadPoolExecutor executor = new ThreadPoolExecutor(
CORE_POOL_SIZE,
MAX_POOL_SIZE,
KEEP_ALIVE_TIME, TimeUnit.SECONDS,
new LinkedBlockingQueue());
allowCoreThreadTimeout(executor, true);
return executor;
}
/**
* Creates a proper Cached Thread Pool. Tasks will reuse cached threads if available
* or create new threads until the core pool is full. tasks will then be queued. If an
* task cannot be queued, a new thread will be created unless this would exceed max pool
* size, then the task will be rejected. Threads will time out after 1 second.
*
* Core thread timeout is only available on android-9+.
*
* @param threadFactory the factory to use when creating new threads
* @return the newly created thread pool
*/
public static ExecutorService newCachedThreadPool(ThreadFactory threadFactory) {
ThreadPoolExecutor executor = new ThreadPoolExecutor(
CORE_POOL_SIZE,
MAX_POOL_SIZE,
KEEP_ALIVE_TIME, TimeUnit.SECONDS,
new LinkedBlockingQueue(),
threadFactory);
allowCoreThreadTimeout(executor, true);
return executor;
}
/**
* Compatibility helper function for
* {@link java.util.concurrent.ThreadPoolExecutor#allowCoreThreadTimeOut(boolean)}
*
* Only available on android-9+.
*
* @param executor the {@link java.util.concurrent.ThreadPoolExecutor}
* @param value true if should time out, else false
*/
@SuppressLint("NewApi")
public static void allowCoreThreadTimeout(ThreadPoolExecutor executor, boolean value) {
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.GINGERBREAD) {
//允许核心线程超时
executor.allowCoreThreadTimeOut(value);
}
}
/**
* An {@link java.util.concurrent.Executor} that executes tasks on the UI thread.
*/
public static Executor uiThread() {
return INSTANCE.uiThread;
}
/**
* An {@link java.util.concurrent.Executor} that runs tasks on the UI thread.
*/
private static class UIThreadExecutor implements Executor {
@Override
public void execute(Runnable command) {
new Handler(Looper.getMainLooper()).post(command);
}
}
}
说明 newCachedThreadPool 通过设置核心线程数为0，来达到空闲线程的回收。
如果有设置核心线程的数量，并且没有设置ThreadPoolExecutor中属性allowCoreThreadTimeOut为true，则核心线程不会销毁。
总结 设置allowCoreThreadTimeOut为true，在选择线程池的时候，可以newFixedThreadPool来代替newCachedThreadPool
ThreadPoolExecutor执行顺序
- 当线程数小于核心线程数时，创建线程。
- 当线程数大于等于核心线程数，且任务队列未满时，将任务放入任务队列。
- 当线程数大于等于核心线程数，且任务队列已满
3.1. 若线程数小于最大线程数，创建线程
3.2. 若线程数等于最大线程数，抛出异常，拒绝任务