AsyncTask在Android中是很常用的异步线程,那么AsyncTask和Thread有什么区别呢?这里将从源码角度深入理解AsyncTask的设计和工作原理,这里的AsyncTask基于SDK-25

分析知识准备

首先我们来看一个生产者与消费者模型的例子

public class ThreadTest {

    //产品
static class ProductObject{
public volatile static String value; //volatile线程操作变量可见
} //生产者线程
static class Producer extends Thread{
Object lock;
public Producer(Object lock) {
this.lock = lock;
}
@Override
public void run() {
while(true){
synchronized (lock) {
if(ProductObject.value != null){
try {
lock.wait(); //产品还没有被消费,等待
} catch (InterruptedException e) {
e.printStackTrace();
}
}
ProductObject.value = "NO:"+System.currentTimeMillis();
System.out.println("生产产品:"+ProductObject.value);
lock.notify(); //生产完成,通知消费者消费
}
}
}
} //消费者线程
static class Consumer extends Thread{
Object lock;
public Consumer(Object lock) {
this.lock = lock;
}
@Override
public void run() {
while(true){
synchronized (lock) {
if(ProductObject.value == null){
try {
lock.wait(); //等待,阻塞
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println("消费产品:"+ProductObject.value);
ProductObject.value = null;
lock.notify(); //消费完成,通知生产者,继续生产
}
}
}
} public static void main(String[] args) {
Object lock = new Object();
new Producer(lock).start();
new Consumer(lock).start();
}
}

上面的例子关键点在于两个,其一是 volatile,使得线程间可见,第二个点在于互斥锁,这样就可以使得有商品的时候就要通知消费者消费,同时 wait,那么消费者收到消息开始消费,消费完毕通知生产者继续生产,从而不断生产,这样比轮询方式更加节省资源

在了解完上面的例子以后,我们就可以着手分析AsyncTask的源代码了

首先,我们在AsyncTask首先看其构造方法

private final WorkerRunnable<Params, Result> mWorker;
private final FutureTask<Result> mFuture;
···
public AsyncTask() {
mWorker = new WorkerRunnable<Params, Result>() {
public Result call() throws Exception {
mTaskInvoked.set(true);
Result result = null;
try {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
//noinspection unchecked
result = doInBackground(mParams);
Binder.flushPendingCommands();
} catch (Throwable tr) {
mCancelled.set(true);
throw tr;
} finally {
postResult(result);
}
return result;
}
}; mFuture = new FutureTask<Result>(mWorker) {
@Override
protected void done() {
try {
postResultIfNotInvoked(get());
} catch (InterruptedException e) {
android.util.Log.w(LOG_TAG, e);
} catch (ExecutionException e) {
throw new RuntimeException("An error occurred while executing doInBackground()",
e.getCause());
} catch (CancellationException e) {
postResultIfNotInvoked(null);
}
}
};
}

这里首先给WorkerRunnableFuture进行了初始化,那么为何要初始化这两个变量呢?

这里就要说到常用的两个方法了,doInBackground(),这个方法是在子线程里面完成的,另一个方法就是onPostExecute(),而这个方法是存在于主线程的,那么也就是说子线程执行完将执行的结果传递到了主线程中,实现了线程间的通信,那么最关键的问题来了,这个通信是怎么实现的呢?
通常在子线程中执行的任务,是没有返回结果的,例如Runnable的源代码如下,就没有返回结果

public interface Runnable {
/**
* When an object implementing interface <code>Runnable</code> is used
* to create a thread, starting the thread causes the object's
* <code>run</code> method to be called in that separately executing
* thread.
* <p>
* The general contract of the method <code>run</code> is that it may
* take any action whatsoever.
*
* @see java.lang.Thread#run()
*/
public abstract void run();
}

那么,要怎么才能得到返回值呢,这里首先想到的就是Callable接口,那么再看看Callable的源代码

@FunctionalInterface
public interface Callable<V> {
/**
* Computes a result, or throws an exception if unable to do so.
*
* @return computed result
* @throws Exception if unable to compute a result
*/
V call() throws Exception;
}

可以看到,这是一个泛型方法,是有返回值的,但是其本身确是不能直接执行的,需要借助其他类,接下来再看一看源代码中涉及到的Future接口

public interface Future<V> {

    /**
* Attempts to cancel execution of this task. This attempt will
* fail if the task has already completed, has already been cancelled,
* or could not be cancelled for some other reason. If successful,
* and this task has not started when {@code cancel} is called,
* this task should never run. If the task has already started,
* then the {@code mayInterruptIfRunning} parameter determines
* whether the thread executing this task should be interrupted in
* an attempt to stop the task.
*
* <p>After this method returns, subsequent calls to {@link #isDone} will
* always return {@code true}. Subsequent calls to {@link #isCancelled}
* will always return {@code true} if this method returned {@code true}.
*
* @param mayInterruptIfRunning {@code true} if the thread executing this
* task should be interrupted; otherwise, in-progress tasks are allowed
* to complete
* @return {@code false} if the task could not be cancelled,
* typically because it has already completed normally;
* {@code true} otherwise
*/
boolean cancel(boolean mayInterruptIfRunning); /**
* Returns {@code true} if this task was cancelled before it completed
* normally.
*
* @return {@code true} if this task was cancelled before it completed
*/
boolean isCancelled(); /**
* Returns {@code true} if this task completed.
*
* Completion may be due to normal termination, an exception, or
* cancellation -- in all of these cases, this method will return
* {@code true}.
*
* @return {@code true} if this task completed
*/
boolean isDone(); /**
* Waits if necessary for the computation to complete, and then
* retrieves its result.
*
* @return the computed result
* @throws CancellationException if the computation was cancelled
* @throws ExecutionException if the computation threw an
* exception
* @throws InterruptedException if the current thread was interrupted
* while waiting
*/
V get() throws InterruptedException, ExecutionException; /**
* Waits if necessary for at most the given time for the computation
* to complete, and then retrieves its result, if available.
*
* @param timeout the maximum time to wait
* @param unit the time unit of the timeout argument
* @return the computed result
* @throws CancellationException if the computation was cancelled
* @throws ExecutionException if the computation threw an
* exception
* @throws InterruptedException if the current thread was interrupted
* while waiting
* @throws TimeoutException if the wait timed out
*/
V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}

Future类中,有好几个方法,而这些方法都是有返回值的,那么RunnableFutureFutureTask有什么关系呢,产看源码便可得知,FutureTask实际上是实现了RunnableFuture接口

public class FutureTask<V> implements RunnableFuture<V>{ ··· }

RunnableFuture又继承了RunnableFuture

public interface RunnableFuture<V> extends Runnable, Future<V> { void run(); }

那也就是说,FutureTask既可以在子线程中执行,也可以获得执行结果,下面使用一个例子来说明FutureTask

public class FutureTest {

    public static void main(String[] args) {
Task work = new Task();
FutureTask<Integer> future = new FutureTask<Integer>(work){
@Override
protected void done() { //异步任务执行完成,回调
try {
System.out.println("done:" + get()); //get()获取异步任务的返回值,这是个阻塞方法
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
}
};
//线程池(使用了预定义的配置)
ExecutorService executor = Executors.newCachedThreadPool();
executor.execute(future);
} //异步任务
static class Task implements Callable<Integer>{ @Override
public Integer call() throws Exception {//返回异步任务的执行结果
int i = 0;
for (; i < 10; i++) {
try {
System.out.println(Thread.currentThread().getName() + "_" + i);
Thread.sleep(500);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
return i;
}
}
}

上面的例子可以看出,在使用了Callable的时候,需要借助FutureTask来包装,然后使用Executorexecute()方法来执行,那么是怎么得到异步任务的返回值呢,在上面的例子中,我们可以看到,其返回值的获取是通过future.get()得到的,然而这个get()方法确是被阻塞的,只有在异步任务完成的时候才能获取到其结果,那我们怎么才能知道异步任务时候执行完毕呢,这里就可以实现FutureTaskdone()方法,当异步任务执行完毕以后会回调这个方法,上述例子其实解释了AsyncTask的实现逻辑,call()方法是在子线程中完成,这也就是doInBackground()的实现,在主线程中获得结果,这是在onPostExecute()使用了get()方法,那也就是说AsyncTask就是通过这一套方法去实现的

从这里我们可以总结出FutureTask为异步任务提供了诸多便利性,包括

  1. 获取异步任务的返回值
  2. 监听异步任务的执行情况
  3. 取消异步任务

那么在AsyncTask中,WorkerRunnable又是啥呢,其实就是一个内部类,对Callable进行了封装

private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> { Params[] mParams; }

源代码分析

有了以上知识储备,我们就可以动手分析AsyncTask源代码了
拿到源代码,不同的人有不同的分析习惯,这里我按照我的习惯对源代码进行一次分析

构造方法分析

首先,因为我们分析源代码是为了更好的去使用,而使用的话,第一个关注的就应该是构造方法,回到之前的的构造方法,这里要开始对构造方法开始入手分析了

private final WorkerRunnable<Params, Result> mWorker;
private final FutureTask<Result> mFuture;
private final AtomicBoolean mCancelled = new AtomicBoolean();
private final AtomicBoolean mTaskInvoked = new AtomicBoolean();
···
public AsyncTask() {
mWorker = new WorkerRunnable<Params, Result>() {
public Result call() throws Exception {
//设置线程调用
mTaskInvoked.set(true);
Result result = null;
try {
//设置线程优先级,其给定值为10
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
//调用doInBackground()方法得到返回值
result = doInBackground(mParams);
//将当前线程中的Binder命令发送至kernel
Binder.flushPendingCommands();
} catch (Throwable tr) {
//发生异常则取消线程调用设置
mCancelled.set(true);
throw tr;
} finally {
//执行postResult()方法
postResult(result);
}
return result;
}
}; mFuture = new FutureTask<Result>(mWorker) {
@Override
protected void done() {
try {
//执行postResultIfNotInvoked()方法
postResultIfNotInvoked(get());
} catch (InterruptedException e) {
android.util.Log.w(LOG_TAG, e);
} catch (ExecutionException e) {
throw new RuntimeException("An error occurred while executing doInBackground()",
e.getCause());
} catch (CancellationException e) {
postResultIfNotInvoked(null);
}
}
};
}

以上就是AsyncTask的构造方法了,在构造方法上有一句说明,这个构造方法必须在UI线程中创建,这一点很好理解,因为其有需要再主线程中执行的地方,后面会说到,那么这个构造方法干了什么事情呢,很简单,这里新建了两个对象,首先是WorkerRunnable,而这个WorkerRunnable则是实现自Callable接口,主要是要使用其call()方法,为了返回参数,并没有什么特别之处,再其内部则实现了call()方法,而其自生是无法执行的,需要找一个包装类,而这个包装类就是FutureTask,通过之前的分析,这里就不再多赘述关于FutureTask的东西了,这里实现了done()方法,也就是线程执行完毕调用的方法,简单点来说就是在call()方法中执行,在done()中获得执行的返回结果,上述涉及到一个内部类和三个自定义的方法,那么接下来我们看一看这个内部类和三个方法都干了啥

构造方法中出现的内部类

这里的WorkerRunnable,正如前面所说,这里除了实现Callable就啥也没干,还是个抽象方法,这里将实现放在了构造方法中

private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> { Params[] mParams; }

构造方法中调用的方法

首先是doInBackground()方法,前面讲到,这个方法在子线程中完成,那么这里的子线程是哪个呢,其实就是WorkerThread,这个方法是一个抽象方法,放在子线程中执行,其具体实现由调用者完成

@WorkerThread protected abstract Result doInBackground(Params... params);

再看postResult()方法,这里获取了一个Handler,然后发送了一个消息,这里就是子线程能够通信主线程的地方了

private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(this, result));
message.sendToTarget();
return result;
}

那么到这里,我们关注的重点就来了,子线程是怎么告诉主线程的呢,要知道其中的原因,我们就需要去看看代码里面是怎么实现的
我们先看发送了什么消息,也就是AsyncTaskResult里面干了啥,查看代码发现,其就是做了参数传递的任务

@SuppressWarnings({"RawUseOfParameterizedType"})
private static class AsyncTaskResult<Data> {
final AsyncTask mTask;
final Data[] mData; AsyncTaskResult(AsyncTask task, Data... data) {
mTask = task;
mData = data;
}
}

那么接下来的重点就是getHandler()方法了,这里拿到AsyncTask.class就上了锁了,这也很好理解,不上锁其他线程走到这里会产生安全隐患,然后返回sHandler,那再继续看看InternalHandler又是个什么吧

private static InternalHandler sHandler;
···
private static Handler getHandler() {
synchronized (AsyncTask.class) {
if (sHandler == null) {
sHandler = new InternalHandler();
}
return sHandler;
}
}

这里就是了,在构造方法里面super(Looper.getMainLooper()),也就说明了这个方法是在主线程中执行的,在主线程中对Message进行处理,这里又涉及到两个方法,一个是finish(),还有一个是onProgressUpdate(),那么好吧,再去看看这两个方法在干啥

private static final int MESSAGE_POST_RESULT = 0x1;
private static final int MESSAGE_POST_PROGRESS = 0x2;
···
private static class InternalHandler extends Handler {
public InternalHandler() {
super(Looper.getMainLooper());
} @SuppressWarnings({"unchecked", "RawUseOfParameterizedType"})
@Override
public void handleMessage(Message msg) {
AsyncTaskResult<?> result = (AsyncTaskResult<?>) msg.obj;
switch (msg.what) {
case MESSAGE_POST_RESULT:
// There is only one result
result.mTask.finish(result.mData[0]);
break;
case MESSAGE_POST_PROGRESS:
result.mTask.onProgressUpdate(result.mData);
break;
}
}
}

首先是又调了isCancelled(),判断是否取消,前面构造函数的时候见过这个,这是在异常发生的时候才设置为true的,那么如果不发生异常,这里应该就是为false的,但在找源代码时发现,另一个方法也对这个参数进行了设置,那就是cancel(),所以在不发生异常和取消的时候应该是为true的,接下来是onCancelled()方法,这里是不做任何操作的,这也是主线程中的方法,还有就是onPostExecute()方法,然后会设置状态,其默认状态是PENDING

private volatile Status mStatus = Status.PENDING;
···
public enum Status {
PENDING,
RUNNING,
FINISHED,
}
···
private void finish(Result result) {
if (isCancelled()) {
onCancelled(result);
} else {
onPostExecute(result);
}
mStatus = Status.FINISHED;
}
@SuppressWarnings({"UnusedDeclaration"})
@MainThread
protected void onPostExecute(Result result) {
} @SuppressWarnings({"UnusedParameters"})
@MainThread
protected void onCancelled(Result result) {
onCancelled();
} @MainThread
protected void onCancelled() {
} public final boolean isCancelled() {
return mCancelled.get();
}
···
public final boolean cancel(boolean mayInterruptIfRunning) {
mCancelled.set(true);
return mFuture.cancel(mayInterruptIfRunning);
}

然后是onProgressUpdate()方法,那么这里做了啥呢,嗯~啥也没有,交给调用者在继承时可以使用

@SuppressWarnings({"UnusedDeclaration"})
@MainThread
protected void onProgressUpdate(Progress... values) {
}

来看构造方法中涉及到的最后一个方法postResultIfNotInvoked(),这个方法又干了啥了,首先获得了mTaskInvoked的状态,整个AsyncTask只有构造方法处设置了这个值,然后判断是否执行postResult()方法

private void postResultIfNotInvoked(Result result) {
final boolean wasTaskInvoked = mTaskInvoked.get();
if (!wasTaskInvoked) {
postResult(result);
}
}

至此构造方法分析完成,可以看到在构造方法中,其主要做的工作最主要的就是搭建好了子线程和主线程沟通的桥梁

执行入口分析

在新建AsyncTask对象以后,要执行的话,需要使用execute()去开始执行
那么我们就从这里入手,看看其具体是怎么工作的,可以看到无论是构造方法还是启动方法,都是需要在主线程中完成的,在execute()中,做了些什么呢,在这之前我们先看看传递的sDefaultExecutor是啥

@MainThread
public final AsyncTask<Params, Progress, Result> execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);
}

这其实是一个线程池,任务调度的线程池,可以看到SerialExecutor实际上是实现了Executor接口,其作用就是将任务添加到双向队列,然后不断地取出执行取出执行,那么THREAD_POOL_EXECUTOR也应该是一个线程池,那这又是啥呢,去看一看这个玩意儿就是到了

public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
···
private static class SerialExecutor implements Executor {
//定义了一个双向队列,用来存储线程
final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();
Runnable mActive; public synchronized void execute(final Runnable r) {
//向队列中添加线程
mTasks.offer(new Runnable() {
public void run() {
try {
//线程运行
r.run();
} finally {
//执行scheduleNext()方法
scheduleNext();
}
}
});
if (mActive == null) {
scheduleNext();
}
} //从队列中取出线程并执行
protected synchronized void scheduleNext() {
if ((mActive = mTasks.poll()) != null) {
THREAD_POOL_EXECUTOR.execute(mActive);
}
}
}

下列代码就是初始化了线程池的参数,指定了线程数量

//获得可用CPU数量
private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
//设置核心线程池数量其范围[2,4],无论是否使用都存在
private static final int CORE_POOL_SIZE = Math.max(2, Math.min(CPU_COUNT - 1, 4));
//设置最大线程数量
private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
//设置闲置回收时间,也就是说线程在这个时间内没有活动的话,会被回收
private static final int KEEP_ALIVE_SECONDS = 30;
//设置线程工厂,通过这个创建线程
private static final ThreadFactory sThreadFactory = new ThreadFactory() {
//创建线程安全的线程个数计数器
private final AtomicInteger mCount = new AtomicInteger(1); public Thread newThread(Runnable r) {
return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());
}
};
//设置任务队列大小
private static final BlockingQueue<Runnable> sPoolWorkQueue =
new LinkedBlockingQueue<Runnable>(128);
//设置线程池
public static final Executor THREAD_POOL_EXECUTOR; //初始化线程池
static {
ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(
CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE_SECONDS, TimeUnit.SECONDS,
sPoolWorkQueue, sThreadFactory);
//打开核心线程池的超时时间
threadPoolExecutor.allowCoreThreadTimeOut(true);
THREAD_POOL_EXECUTOR = threadPoolExecutor;
}

所以在执行scheduleNext()的时候,会将THREAD_POOL_EXECUTOR中设置好的线程全部取出来,用来执行后面的任务,其执行的任务就是execute()方法所指定的任务,在executeOnExecutor()方法中,由于前面初始化完成,这里的状态应该是PENDING,之后还设置了mWorker的参数,然后会执行线程池的方法,然后据开始执行任务了,前面没有涉及到的方法还有一个,那我们接下来看看

@MainThread
public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
Params... params) {
if (mStatus != Status.PENDING) {
switch (mStatus) {
case RUNNING:
throw new IllegalStateException("Cannot execute task:"
+ " the task is already running.");
case FINISHED:
throw new IllegalStateException("Cannot execute task:"
+ " the task has already been executed "
+ "(a task can be executed only once)");
}
}
mStatus = Status.RUNNING;
onPreExecute();
mWorker.mParams = params;
exec.execute(mFuture);
return this;
}

onPreExecute(),这个方法由调用者在继承时候能够使用

@MainThread
protected void onPreExecute() {
}

至此,AsyncTask的执行方法也分析完了,那么我们接下来看看还有什么方法没有涉及到,没有涉及到的方法都是public属性和方法

AsyncTask的公共方法

//这个方法设置为public,那么就意味着我们可以自定义线程池
public static void setDefaultExecutor(Executor exec) {
sDefaultExecutor = exec;
}
//这个方法意味着我们可以获得其状态,配合枚举值使用
public enum Status {
PENDING,
RUNNING,
FINISHED,
}
···
public final Status getStatus() {
return mStatus;
}
//查看是非被取消
public final boolean isCancelled() {
return mCancelled.get();
}
//取消异步任务
public final boolean cancel(boolean mayInterruptIfRunning) {
mCancelled.set(true);
return mFuture.cancel(mayInterruptIfRunning);
}
//获取返回结果,注意:这个方法是阻塞式的
public final Result get() throws InterruptedException, ExecutionException {
return mFuture.get();
}
//同上
public final Result get(long timeout, TimeUnit unit) throws InterruptedException,
ExecutionException, TimeoutException {
return mFuture.get(timeout, unit);
}
//自定义的线程池可以从这个方法启动
@MainThread
public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
Params... params) {
if (mStatus != Status.PENDING) {
switch (mStatus) {
case RUNNING:
throw new IllegalStateException("Cannot execute task:"
+ " the task is already running.");
case FINISHED:
throw new IllegalStateException("Cannot execute task:"
+ " the task has already been executed "
+ "(a task can be executed only once)");
}
}
mStatus = Status.RUNNING;
onPreExecute();
mWorker.mParams = params;
exec.execute(mFuture);
return this;
}
//使用默认线程池启动异步任务
@MainThread
public static void execute(Runnable runnable) {
sDefaultExecutor.execute(runnable);
}

总结

AsyncTask的实例化过程,其本质上就是实例化了一个FutureTask

其执行过程Executor.execute(mFuture) -> SerialExecutor.mTasks(队列) -> (线程池)THREAD_POOL_EXECUTOR.execute

线程池中的所有线程,为了执行异步任务

如果当前线程池中的数量小于corePoolSize,创建并添加的任务
如果当前线程池中的数量等于corePoolSize,缓冲队列workQueue未满,那么任务被放入缓冲队列、等待任务调度执行
如果当前线程池中的数量大于corePoolSize,缓冲队列workQueue已满,并且线程池中的数量小于maximumPoolSize,新提交任务会创建新线程执行任务
如果当前线程池中的数量大于corePoolSize,缓冲队列workQueue已满,并且线程池中的数量等于maximumPoolSize,新提交任务由Handler处理
当线程池中的线程大于corePoolSize时,多余线程空闲时间超过keepAliveTime时,会关闭这部分线程

线程池在添加时候是串行的,在执行任务的时候是并行的

附录(源代码)

package android.os;

import android.annotation.MainThread;
import android.annotation.WorkerThread; import java.util.ArrayDeque;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.Callable;
import java.util.concurrent.CancellationException;
import java.util.concurrent.Executor;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.FutureTask;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicInteger; /**
* <p>AsyncTask enables proper and easy use of the UI thread. This class allows you
* to perform background operations and publish results on the UI thread without
* having to manipulate threads and/or handlers.</p>
*
* <p>AsyncTask is designed to be a helper class around {@link Thread} and {@link Handler}
* and does not constitute a generic threading framework. AsyncTasks should ideally be
* used for short operations (a few seconds at the most.) If you need to keep threads
* running for long periods of time, it is highly recommended you use the various APIs
* provided by the <code>java.util.concurrent</code> package such as {@link Executor},
* {@link ThreadPoolExecutor} and {@link FutureTask}.</p>
*
* <p>An asynchronous task is defined by a computation that runs on a background thread and
* whose result is published on the UI thread. An asynchronous task is defined by 3 generic
* types, called <code>Params</code>, <code>Progress</code> and <code>Result</code>,
* and 4 steps, called <code>onPreExecute</code>, <code>doInBackground</code>,
* <code>onProgressUpdate</code> and <code>onPostExecute</code>.</p>
*
* <div class="special reference">
* <h3>Developer Guides</h3>
* <p>For more information about using tasks and threads, read the
* <a href="{@docRoot}guide/components/processes-and-threads.html">Processes and
* Threads</a> developer guide.</p>
* </div>
*
* <h2>Usage</h2>
* <p>AsyncTask must be subclassed to be used. The subclass will override at least
* one method ({@link #doInBackground}), and most often will override a
* second one ({@link #onPostExecute}.)</p>
*
* <p>Here is an example of subclassing:</p>
* <pre class="prettyprint">
* private class DownloadFilesTask extends AsyncTask&lt;URL, Integer, Long&gt; {
* protected Long doInBackground(URL... urls) {
* int count = urls.length;
* long totalSize = 0;
* for (int i = 0; i < count; i++) {
* totalSize += Downloader.downloadFile(urls[i]);
* publishProgress((int) ((i / (float) count) * 100));
* // Escape early if cancel() is called
* if (isCancelled()) break;
* }
* return totalSize;
* }
*
* protected void onProgressUpdate(Integer... progress) {
* setProgressPercent(progress[0]);
* }
*
* protected void onPostExecute(Long result) {
* showDialog("Downloaded " + result + " bytes");
* }
* }
* </pre>
*
* <p>Once created, a task is executed very simply:</p>
* <pre class="prettyprint">
* new DownloadFilesTask().execute(url1, url2, url3);
* </pre>
*
* <h2>AsyncTask's generic types</h2>
* <p>The three types used by an asynchronous task are the following:</p>
* <ol>
* <li><code>Params</code>, the type of the parameters sent to the task upon
* execution.</li>
* <li><code>Progress</code>, the type of the progress units published during
* the background computation.</li>
* <li><code>Result</code>, the type of the result of the background
* computation.</li>
* </ol>
* <p>Not all types are always used by an asynchronous task. To mark a type as unused,
* simply use the type {@link Void}:</p>
* <pre>
* private class MyTask extends AsyncTask&lt;Void, Void, Void&gt; { ... }
* </pre>
*
* <h2>The 4 steps</h2>
* <p>When an asynchronous task is executed, the task goes through 4 steps:</p>
* <ol>
* <li>{@link #onPreExecute()}, invoked on the UI thread before the task
* is executed. This step is normally used to setup the task, for instance by
* showing a progress bar in the user interface.</li>
* <li>{@link #doInBackground}, invoked on the background thread
* immediately after {@link #onPreExecute()} finishes executing. This step is used
* to perform background computation that can take a long time. The parameters
* of the asynchronous task are passed to this step. The result of the computation must
* be returned by this step and will be passed back to the last step. This step
* can also use {@link #publishProgress} to publish one or more units
* of progress. These values are published on the UI thread, in the
* {@link #onProgressUpdate} step.</li>
* <li>{@link #onProgressUpdate}, invoked on the UI thread after a
* call to {@link #publishProgress}. The timing of the execution is
* undefined. This method is used to display any form of progress in the user
* interface while the background computation is still executing. For instance,
* it can be used to animate a progress bar or show logs in a text field.</li>
* <li>{@link #onPostExecute}, invoked on the UI thread after the background
* computation finishes. The result of the background computation is passed to
* this step as a parameter.</li>
* </ol>
*
* <h2>Cancelling a task</h2>
* <p>A task can be cancelled at any time by invoking {@link #cancel(boolean)}. Invoking
* this method will cause subsequent calls to {@link #isCancelled()} to return true.
* After invoking this method, {@link #onCancelled(Object)}, instead of
* {@link #onPostExecute(Object)} will be invoked after {@link #doInBackground(Object[])}
* returns. To ensure that a task is cancelled as quickly as possible, you should always
* check the return value of {@link #isCancelled()} periodically from
* {@link #doInBackground(Object[])}, if possible (inside a loop for instance.)</p>
*
* <h2>Threading rules</h2>
* <p>There are a few threading rules that must be followed for this class to
* work properly:</p>
* <ul>
* <li>The AsyncTask class must be loaded on the UI thread. This is done
* automatically as of {@link android.os.Build.VERSION_CODES#JELLY_BEAN}.</li>
* <li>The task instance must be created on the UI thread.</li>
* <li>{@link #execute} must be invoked on the UI thread.</li>
* <li>Do not call {@link #onPreExecute()}, {@link #onPostExecute},
* {@link #doInBackground}, {@link #onProgressUpdate} manually.</li>
* <li>The task can be executed only once (an exception will be thrown if
* a second execution is attempted.)</li>
* </ul>
*
* <h2>Memory observability</h2>
* <p>AsyncTask guarantees that all callback calls are synchronized in such a way that the following
* operations are safe without explicit synchronizations.</p>
* <ul>
* <li>Set member fields in the constructor or {@link #onPreExecute}, and refer to them
* in {@link #doInBackground}.
* <li>Set member fields in {@link #doInBackground}, and refer to them in
* {@link #onProgressUpdate} and {@link #onPostExecute}.
* </ul>
*
* <h2>Order of execution</h2>
* <p>When first introduced, AsyncTasks were executed serially on a single background
* thread. Starting with {@link android.os.Build.VERSION_CODES#DONUT}, this was changed
* to a pool of threads allowing multiple tasks to operate in parallel. Starting with
* {@link android.os.Build.VERSION_CODES#HONEYCOMB}, tasks are executed on a single
* thread to avoid common application errors caused by parallel execution.</p>
* <p>If you truly want parallel execution, you can invoke
* {@link #executeOnExecutor(java.util.concurrent.Executor, Object[])} with
* {@link #THREAD_POOL_EXECUTOR}.</p>
*/
public abstract class AsyncTask<Params, Progress, Result> {
private static final String LOG_TAG = "AsyncTask"; private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
// We want at least 2 threads and at most 4 threads in the core pool,
// preferring to have 1 less than the CPU count to avoid saturating
// the CPU with background work
private static final int CORE_POOL_SIZE = Math.max(2, Math.min(CPU_COUNT - 1, 4));
private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
private static final int KEEP_ALIVE_SECONDS = 30; private static final ThreadFactory sThreadFactory = new ThreadFactory() {
private final AtomicInteger mCount = new AtomicInteger(1); public Thread newThread(Runnable r) {
return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());
}
}; private static final BlockingQueue<Runnable> sPoolWorkQueue =
new LinkedBlockingQueue<Runnable>(128); /**
* An {@link Executor} that can be used to execute tasks in parallel.
*/
public static final Executor THREAD_POOL_EXECUTOR; static {
ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(
CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE_SECONDS, TimeUnit.SECONDS,
sPoolWorkQueue, sThreadFactory);
threadPoolExecutor.allowCoreThreadTimeOut(true);
THREAD_POOL_EXECUTOR = threadPoolExecutor;
} /**
* An {@link Executor} that executes tasks one at a time in serial
* order. This serialization is global to a particular process.
*/
public static final Executor SERIAL_EXECUTOR = new SerialExecutor(); private static final int MESSAGE_POST_RESULT = 0x1;
private static final int MESSAGE_POST_PROGRESS = 0x2; private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
private static InternalHandler sHandler; private final WorkerRunnable<Params, Result> mWorker;
private final FutureTask<Result> mFuture; private volatile Status mStatus = Status.PENDING; private final AtomicBoolean mCancelled = new AtomicBoolean();
private final AtomicBoolean mTaskInvoked = new AtomicBoolean(); private static class SerialExecutor implements Executor {
final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();
Runnable mActive; public synchronized void execute(final Runnable r) {
mTasks.offer(new Runnable() {
public void run() {
try {
r.run();
} finally {
scheduleNext();
}
}
});
if (mActive == null) {
scheduleNext();
}
} protected synchronized void scheduleNext() {
if ((mActive = mTasks.poll()) != null) {
THREAD_POOL_EXECUTOR.execute(mActive);
}
}
} /**
* Indicates the current status of the task. Each status will be set only once
* during the lifetime of a task.
*/
public enum Status {
/**
* Indicates that the task has not been executed yet.
*/
PENDING,
/**
* Indicates that the task is running.
*/
RUNNING,
/**
* Indicates that {@link AsyncTask#onPostExecute} has finished.
*/
FINISHED,
} private static Handler getHandler() {
synchronized (AsyncTask.class) {
if (sHandler == null) {
sHandler = new InternalHandler();
}
return sHandler;
}
} /** @hide */
public static void setDefaultExecutor(Executor exec) {
sDefaultExecutor = exec;
} /**
* Creates a new asynchronous task. This constructor must be invoked on the UI thread.
*/
public AsyncTask() {
mWorker = new WorkerRunnable<Params, Result>() {
public Result call() throws Exception {
mTaskInvoked.set(true);
Result result = null;
try {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
//noinspection unchecked
result = doInBackground(mParams);
Binder.flushPendingCommands();
} catch (Throwable tr) {
mCancelled.set(true);
throw tr;
} finally {
postResult(result);
}
return result;
}
}; mFuture = new FutureTask<Result>(mWorker) {
@Override
protected void done() {
try {
postResultIfNotInvoked(get());
} catch (InterruptedException e) {
android.util.Log.w(LOG_TAG, e);
} catch (ExecutionException e) {
throw new RuntimeException("An error occurred while executing doInBackground()",
e.getCause());
} catch (CancellationException e) {
postResultIfNotInvoked(null);
}
}
};
} private void postResultIfNotInvoked(Result result) {
final boolean wasTaskInvoked = mTaskInvoked.get();
if (!wasTaskInvoked) {
postResult(result);
}
} private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(this, result));
message.sendToTarget();
return result;
} /**
* Returns the current status of this task.
*
* @return The current status.
*/
public final Status getStatus() {
return mStatus;
} /**
* Override this method to perform a computation on a background thread. The
* specified parameters are the parameters passed to {@link #execute}
* by the caller of this task.
*
* This method can call {@link #publishProgress} to publish updates
* on the UI thread.
*
* @param params The parameters of the task.
*
* @return A result, defined by the subclass of this task.
*
* @see #onPreExecute()
* @see #onPostExecute
* @see #publishProgress
*/
@WorkerThread
protected abstract Result doInBackground(Params... params); /**
* Runs on the UI thread before {@link #doInBackground}.
*
* @see #onPostExecute
* @see #doInBackground
*/
@MainThread
protected void onPreExecute() {
} /**
* <p>Runs on the UI thread after {@link #doInBackground}. The
* specified result is the value returned by {@link #doInBackground}.</p>
*
* <p>This method won't be invoked if the task was cancelled.</p>
*
* @param result The result of the operation computed by {@link #doInBackground}.
*
* @see #onPreExecute
* @see #doInBackground
* @see #onCancelled(Object)
*/
@SuppressWarnings({"UnusedDeclaration"})
@MainThread
protected void onPostExecute(Result result) {
} /**
* Runs on the UI thread after {@link #publishProgress} is invoked.
* The specified values are the values passed to {@link #publishProgress}.
*
* @param values The values indicating progress.
*
* @see #publishProgress
* @see #doInBackground
*/
@SuppressWarnings({"UnusedDeclaration"})
@MainThread
protected void onProgressUpdate(Progress... values) {
} /**
* <p>Runs on the UI thread after {@link #cancel(boolean)} is invoked and
* {@link #doInBackground(Object[])} has finished.</p>
*
* <p>The default implementation simply invokes {@link #onCancelled()} and
* ignores the result. If you write your own implementation, do not call
* <code>super.onCancelled(result)</code>.</p>
*
* @param result The result, if any, computed in
* {@link #doInBackground(Object[])}, can be null
*
* @see #cancel(boolean)
* @see #isCancelled()
*/
@SuppressWarnings({"UnusedParameters"})
@MainThread
protected void onCancelled(Result result) {
onCancelled();
} /**
* <p>Applications should preferably override {@link #onCancelled(Object)}.
* This method is invoked by the default implementation of
* {@link #onCancelled(Object)}.</p>
*
* <p>Runs on the UI thread after {@link #cancel(boolean)} is invoked and
* {@link #doInBackground(Object[])} has finished.</p>
*
* @see #onCancelled(Object)
* @see #cancel(boolean)
* @see #isCancelled()
*/
@MainThread
protected void onCancelled() {
} /**
* Returns <tt>true</tt> if this task was cancelled before it completed
* normally. If you are calling {@link #cancel(boolean)} on the task,
* the value returned by this method should be checked periodically from
* {@link #doInBackground(Object[])} to end the task as soon as possible.
*
* @return <tt>true</tt> if task was cancelled before it completed
*
* @see #cancel(boolean)
*/
public final boolean isCancelled() {
return mCancelled.get();
} /**
* <p>Attempts to cancel execution of this task. This attempt will
* fail if the task has already completed, already been cancelled,
* or could not be cancelled for some other reason. If successful,
* and this task has not started when <tt>cancel</tt> is called,
* this task should never run. If the task has already started,
* then the <tt>mayInterruptIfRunning</tt> parameter determines
* whether the thread executing this task should be interrupted in
* an attempt to stop the task.</p>
*
* <p>Calling this method will result in {@link #onCancelled(Object)} being
* invoked on the UI thread after {@link #doInBackground(Object[])}
* returns. Calling this method guarantees that {@link #onPostExecute(Object)}
* is never invoked. After invoking this method, you should check the
* value returned by {@link #isCancelled()} periodically from
* {@link #doInBackground(Object[])} to finish the task as early as
* possible.</p>
*
* @param mayInterruptIfRunning <tt>true</tt> if the thread executing this
* task should be interrupted; otherwise, in-progress tasks are allowed
* to complete.
*
* @return <tt>false</tt> if the task could not be cancelled,
* typically because it has already completed normally;
* <tt>true</tt> otherwise
*
* @see #isCancelled()
* @see #onCancelled(Object)
*/
public final boolean cancel(boolean mayInterruptIfRunning) {
mCancelled.set(true);
return mFuture.cancel(mayInterruptIfRunning);
} /**
* Waits if necessary for the computation to complete, and then
* retrieves its result.
*
* @return The computed result.
*
* @throws CancellationException If the computation was cancelled.
* @throws ExecutionException If the computation threw an exception.
* @throws InterruptedException If the current thread was interrupted
* while waiting.
*/
public final Result get() throws InterruptedException, ExecutionException {
return mFuture.get();
} /**
* Waits if necessary for at most the given time for the computation
* to complete, and then retrieves its result.
*
* @param timeout Time to wait before cancelling the operation.
* @param unit The time unit for the timeout.
*
* @return The computed result.
*
* @throws CancellationException If the computation was cancelled.
* @throws ExecutionException If the computation threw an exception.
* @throws InterruptedException If the current thread was interrupted
* while waiting.
* @throws TimeoutException If the wait timed out.
*/
public final Result get(long timeout, TimeUnit unit) throws InterruptedException,
ExecutionException, TimeoutException {
return mFuture.get(timeout, unit);
} /**
* Executes the task with the specified parameters. The task returns
* itself (this) so that the caller can keep a reference to it.
*
* <p>Note: this function schedules the task on a queue for a single background
* thread or pool of threads depending on the platform version. When first
* introduced, AsyncTasks were executed serially on a single background thread.
* Starting with {@link android.os.Build.VERSION_CODES#DONUT}, this was changed
* to a pool of threads allowing multiple tasks to operate in parallel. Starting
* {@link android.os.Build.VERSION_CODES#HONEYCOMB}, tasks are back to being
* executed on a single thread to avoid common application errors caused
* by parallel execution. If you truly want parallel execution, you can use
* the {@link #executeOnExecutor} version of this method
* with {@link #THREAD_POOL_EXECUTOR}; however, see commentary there for warnings
* on its use.
*
* <p>This method must be invoked on the UI thread.
*
* @param params The parameters of the task.
*
* @return This instance of AsyncTask.
*
* @throws IllegalStateException If {@link #getStatus()} returns either
* {@link AsyncTask.Status#RUNNING} or {@link AsyncTask.Status#FINISHED}.
*
* @see #executeOnExecutor(java.util.concurrent.Executor, Object[])
* @see #execute(Runnable)
*/
@MainThread
public final AsyncTask<Params, Progress, Result> execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);
} /**
* Executes the task with the specified parameters. The task returns
* itself (this) so that the caller can keep a reference to it.
*
* <p>This method is typically used with {@link #THREAD_POOL_EXECUTOR} to
* allow multiple tasks to run in parallel on a pool of threads managed by
* AsyncTask, however you can also use your own {@link Executor} for custom
* behavior.
*
* <p><em>Warning:</em> Allowing multiple tasks to run in parallel from
* a thread pool is generally <em>not</em> what one wants, because the order
* of their operation is not defined. For example, if these tasks are used
* to modify any state in common (such as writing a file due to a button click),
* there are no guarantees on the order of the modifications.
* Without careful work it is possible in rare cases for the newer version
* of the data to be over-written by an older one, leading to obscure data
* loss and stability issues. Such changes are best
* executed in serial; to guarantee such work is serialized regardless of
* platform version you can use this function with {@link #SERIAL_EXECUTOR}.
*
* <p>This method must be invoked on the UI thread.
*
* @param exec The executor to use. {@link #THREAD_POOL_EXECUTOR} is available as a
* convenient process-wide thread pool for tasks that are loosely coupled.
* @param params The parameters of the task.
*
* @return This instance of AsyncTask.
*
* @throws IllegalStateException If {@link #getStatus()} returns either
* {@link AsyncTask.Status#RUNNING} or {@link AsyncTask.Status#FINISHED}.
*
* @see #execute(Object[])
*/
@MainThread
public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
Params... params) {
if (mStatus != Status.PENDING) {
switch (mStatus) {
case RUNNING:
throw new IllegalStateException("Cannot execute task:"
+ " the task is already running.");
case FINISHED:
throw new IllegalStateException("Cannot execute task:"
+ " the task has already been executed "
+ "(a task can be executed only once)");
}
} mStatus = Status.RUNNING; onPreExecute(); mWorker.mParams = params;
exec.execute(mFuture); return this;
} /**
* Convenience version of {@link #execute(Object...)} for use with
* a simple Runnable object. See {@link #execute(Object[])} for more
* information on the order of execution.
*
* @see #execute(Object[])
* @see #executeOnExecutor(java.util.concurrent.Executor, Object[])
*/
@MainThread
public static void execute(Runnable runnable) {
sDefaultExecutor.execute(runnable);
} /**
* This method can be invoked from {@link #doInBackground} to
* publish updates on the UI thread while the background computation is
* still running. Each call to this method will trigger the execution of
* {@link #onProgressUpdate} on the UI thread.
*
* {@link #onProgressUpdate} will not be called if the task has been
* canceled.
*
* @param values The progress values to update the UI with.
*
* @see #onProgressUpdate
* @see #doInBackground
*/
@WorkerThread
protected final void publishProgress(Progress... values) {
if (!isCancelled()) {
getHandler().obtainMessage(MESSAGE_POST_PROGRESS,
new AsyncTaskResult<Progress>(this, values)).sendToTarget();
}
} private void finish(Result result) {
if (isCancelled()) {
onCancelled(result);
} else {
onPostExecute(result);
}
mStatus = Status.FINISHED;
} private static class InternalHandler extends Handler {
public InternalHandler() {
super(Looper.getMainLooper());
} @SuppressWarnings({"unchecked", "RawUseOfParameterizedType"})
@Override
public void handleMessage(Message msg) {
AsyncTaskResult<?> result = (AsyncTaskResult<?>) msg.obj;
switch (msg.what) {
case MESSAGE_POST_RESULT:
// There is only one result
result.mTask.finish(result.mData[0]);
break;
case MESSAGE_POST_PROGRESS:
result.mTask.onProgressUpdate(result.mData);
break;
}
}
} private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {
Params[] mParams;
} @SuppressWarnings({"RawUseOfParameterizedType"})
private static class AsyncTaskResult<Data> {
final AsyncTask mTask;
final Data[] mData; AsyncTaskResult(AsyncTask task, Data... data) {
mTask = task;
mData = data;
}
}
}

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