彻底理解JDK异步
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--《论语》
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从同步到异步如何创建一个新线程如何启动一个新线程如何在新线程运行下面方法？两种方式应该使用哪种？我们干了什么？我们的任务通常不会如此简单封装Runnable实现RunnableTask奇怪的结果？为什么呢？优化思路优化后的RunnableTask还存在的问题解决参数问题解决返回值和异步过程控制Runnable升级为RunnableFuture我需要自己实现吗？Runnable另一个问题来一个更好写逻辑代码的接口CallableCallable接口的优点对RunnableFuture的实现大概就是这样可是我也有没有返回值的业务代码JDK中FutureTask执行有返回值的任务总结
从同步到异步
如何创建一个新线程
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new Thread();
如何启动一个新线程
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new Thread().start();
如何在新线程运行下面方法？
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public class Task {
public static final String HELLO_WORLD = "Hello,World!";
public void say() {
System.out.println(Thread.currentThread().getName() + " say : " + HELLO_WORLD);
}
}
两种方式
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public class MultiThread {
public static void main(String[] args) {
new Thread(new Runnable() {
public void run() {
new Task().say();
}
}).start();
new Thread() {
public void run() {
new Task().say();
};
}.start();
}
}
应该使用哪种？
-
传入Runnable对象
推荐。将需要执行的逻辑封装到Runnable子类中
-
重写run()方法
需要扩展Thread的功能时使用。
我们干了什么？
- 我们执行了异步任务！
- 我们在主线程执行时，把一个任务交给了另一个线程执行。另一个任务执行的时候不会阻塞当前流程！
我们的任务通常不会如此简单
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public class Task {
public String say(String param) {
try {
TimeUnit.SECONDS.sleep(10);
} catch (InterruptedException e) {
e.printStackTrace();
}
String result = Thread.currentThread().getName() + " say : " + param;
System.out.println(result);
return result;
}
}
- sleep模拟任务耗时
- 有参数，有返回值的任务如何执行呢？
- 我们需要在主线程中指定不同的参数，同时获取到不同的结果
封装Runnable实现RunnableTask
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public class RunnableTask implements Runnable {
private Task task;
private String param;
private volatile String result;
public RunnableTask(Task task, String param) {
this.task = task;
this.param = param;
}
public void run() {
this.result = task.say(param);
}
public String getResult() {
return result;
}
}
- volatile用于排除线程可见性影响
奇怪的结果？
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public static void main(String[] args) throws Exception {
RunnableTask runnableTask1 = new RunnableTask(new Task(), "Hello,World!");
new Thread(runnableTask1).start();
RunnableTask runnableTask2 = new RunnableTask(new Task(), "Hello,zby!");
new Thread(runnableTask2).start();
// TimeUnit.SECONDS.sleep(15);
System.out.println(Thread.currentThread().getName() + " say : " + runnableTask1.getResult());
System.out.println(Thread.currentThread().getName() + " say : " + runnableTask2.getResult());
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main say : null
Thread-0 say : Hello,World!
main say : null
Thread-1 say : Hello,zby!
- 两个10s的任务，通过异步的方式10s左右就能完成
- 我没有获取到异步执行的结果
- 去掉注释的一行试试？
为什么呢？
- 我们的任务是异步执行的，执行打印结果的时候，我们异步任务可能还没执行完成，因此result尚未设置
- 主线程等待15s，异步线程都执行完毕，可以获取到异步执行结果
优化思路
- 任务本身是异步执行的，但是需要在主线程获取执行结果，那么主线程就需要阻塞等待执行完成
优化后的RunnableTask
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public class RunnableTask implements Runnable {
private Task task;
private String param;
private volatile String result;
private volatile boolean complated = false;
public RunnableTask(Task task, String param) {
this.task = task;
this.param = param;
}
public void run() {
synchronized (this) {
this.result = task.say(param);
complated = true;
this.notifyAll();
}
}
public String getResult() {
synchronized (this) {
while (!complated) {
try {
this.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
return result;
}
}
- 获取结果的时候需要判断是否完成，没完成需要阻塞等待完成
- 使用新的标记complated而不判断result是否为null是因为，result可能执行完返回null
还存在的问题
- 同步机制性能很低，编码灵活度低，如取消等操作无法响应
- 无法获取或者控制异步执行过程
- 我不想对每个任务都进行封装
- 我的参数和返回值类型不确定
解决参数问题
- 可以看到我们的优化代码并没有对参数做处理，其实我们创建Runnable子类的时候就可以指定有参构造方法，并不需要额外处理
解决返回值和异步过程控制
- 抽象出执行结果获取和执行控制过程的接口
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package java.util.concurrent;
public interface Future<V> {
boolean cancel(boolean mayInterruptIfRunning);
boolean isCancelled();
boolean isDone();
V get() throws InterruptedException, ExecutionException;
V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}
- 这个接口是jdk提供的，可以满足普通需求
- 但是，功能有限，只能阻塞方式获取结果。如Netty对它进行了更丰富的扩展，使用观察者模式监听执行结果
Runnable升级为RunnableFuture
- 需要获取异步结果的Runnable子类，应该同时实现Runnable接口和Future接口，并提供对于方法的实现
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package java.util.concurrent;
public interface RunnableFuture<V> extends Runnable, Future<V> {
void run();
}
- 我们不需要每次都去实现两个接口
- 这个接口也是JDK提供的，可以执行任务并获取结果
我需要自己实现吗？
- Future接口提供的功能，是跟业务代码无关的
- run()方法执行的逻辑应该是独特的
- 我们可以使用装饰模式对Runnable进行实现。
- 可以对Future提供的方法进行统一的实现
Runnable另一个问题
- 我们在Runnable接口的run()方法中写自己的逻辑代码，没办法直接返回结果
- 我们装饰的是Runnable接口，该接口无法获取返回值，需要从它的内部获取返回值
- 为了获取到我们任务执行的结果，可能需要这么做
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public interface ResultRunnable<V> extends Runnable {
V getResult();
}
public class MyRunnableFuture<V> implements Runnable, Future<V> {
private ResultRunnable<V> resultRunnable;
public MyRunnableFuture(ResultRunnable<V> resultRunnable) {
this.resultRunnable = resultRunnable;
}
public boolean cancel(boolean mayInterruptIfRunning) {
// TODO Auto-generated method stub
return false;
}
public boolean isCancelled() {
// TODO Auto-generated method stub
return false;
}
public boolean isDone() {
// TODO Auto-generated method stub
return false;
}
public V get() throws InterruptedException, ExecutionException {
// TODO Auto-generated method stub
// 阻塞直到执行完成
return null;
}
public V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException {
// TODO Auto-generated method stub
return null;
}
public void run() {
resultRunnable.run();
resultRunnable.getResult();//获取结果并设置到当前的属性，以便get()能获取
// 通知阻塞结束
}
}
- 这样编码的话，我们需要手动设置执行结果
- 或者抽象结果的设置，我们执行完调用set设置，还是很麻烦，忘了咋办
来一个更好写逻辑代码的接口Callable
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package java.util.concurrent;
public interface Callable<V> {
V call() throws Exception;
}
- 这个接口就符合我们编写业务代码的习惯
Callable接口的优点
- 对Callable接口进行装饰，可以调用执行业务代码，直接获取到返回值
- 无需对业务代码再次封装
对RunnableFuture的实现大概就是这样
class FutureTask<V> implements RunnableFuture<V> {
private Callable<V> callable;
public FutureTask(Callable<V> callable) {
this.callable = callable;
}
public boolean cancel(boolean mayInterruptIfRunning) {
// TODO Auto-generated method stub
return false;
}
public boolean isCancelled() {
// TODO Auto-generated method stub
return false;
}
public boolean isDone() {
// TODO Auto-generated method stub
return false;
}
public V get() throws InterruptedException, ExecutionException {
// TODO Auto-generated method stub
return null;
}
public V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException {
// TODO Auto-generated method stub
return null;
}
public void run() {
try {
V call = callable.call();// 把结果设置到当前类的字段中
// 唤醒等待获取线程
} catch (Exception e) {
e.printStackTrace();
}
}
}
可是我也有没有返回值的业务代码
- 显然Callable比Runnable更强大，多一个返回值
- 为了兼容Runnable，可以使用适配器模式进行封装
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class RunnableAdapter<T> implements Callable<T> {
final Runnable task;
final T result;
RunnableAdapter(Runnable task, T result) {
this.task = task;
this.result = result;
}
public T call() {
task.run();
return result;
}
}
- Runnable接口可以完美转换为Callable
JDK中FutureTask
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package java.util.concurrent;
public class FutureTask<V> implements RunnableFuture<V> {
/*
* Revision notes: This differs from previous versions of this
* class that relied on AbstractQueuedSynchronizer, mainly to
* avoid surprising users about retaining interrupt status during
* cancellation races. Sync control in the current design relies
* on a "state" field updated via CAS to track completion, along
* with a simple Treiber stack to hold waiting threads.
*
* Style note: As usual, we bypass overhead of using
* AtomicXFieldUpdaters and instead directly use Unsafe intrinsics.
*/
/**
* The run state of this task, initially NEW. The run state
* transitions to a terminal state only in methods set,
* setException, and cancel. During completion, state may take on
* transient values of COMPLETING (while outcome is being set) or
* INTERRUPTING (only while interrupting the runner to satisfy a
* cancel(true)). Transitions from these intermediate to final
* states use cheaper ordered/lazy writes because values are unique
* and cannot be further modified.
*
* Possible state transitions:
* NEW -> COMPLETING -> NORMAL
* NEW -> COMPLETING -> EXCEPTIONAL
* NEW -> CANCELLED
* NEW -> INTERRUPTING -> INTERRUPTED
*/
private volatile int state;
private static final int NEW = 0;
private static final int COMPLETING = 1;
private static final int NORMAL = 2;
private static final int EXCEPTIONAL = 3;
private static final int CANCELLED = 4;
private static final int INTERRUPTING = 5;
private static final int INTERRUPTED = 6;
/** The underlying callable; nulled out after running */
private Callable<V> callable;
/** The result to return or exception to throw from get() */
private Object outcome; // non-volatile, protected by state reads/writes
/** The thread running the callable; CASed during run() */
private volatile Thread runner;
/** Treiber stack of waiting threads */
private volatile WaitNode waiters;
/**
* Returns result or throws exception for completed task.
*
* @param s completed state value
*/
("unchecked")
private V report(int s) throws ExecutionException {
Object x = outcome;
if (s == NORMAL)
return (V)x;
if (s >= CANCELLED)
throw new CancellationException();
throw new ExecutionException((Throwable)x);
}
/**
* Creates a {@code FutureTask} that will, upon running, execute the
* given {@code Callable}.
*
* @param callable the callable task
* @throws NullPointerException if the callable is null
*/
public FutureTask(Callable<V> callable) {
if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW; // ensure visibility of callable
}
/**
* Creates a {@code FutureTask} that will, upon running, execute the
* given {@code Runnable}, and arrange that {@code get} will return the
* given result on successful completion.
*
* @param runnable the runnable task
* @param result the result to return on successful completion. If
* you don't need a particular result, consider using
* constructions of the form:
* {@code Future<?> f = new FutureTask<Void>(runnable, null)}
* @throws NullPointerException if the runnable is null
*/
public FutureTask(Runnable runnable, V result) {
this.callable = Executors.callable(runnable, result);
this.state = NEW; // ensure visibility of callable
}
public boolean isCancelled() {
return state >= CANCELLED;
}
public boolean isDone() {
return state != NEW;
}
public boolean cancel(boolean mayInterruptIfRunning) {
if (!(state == NEW &&
UNSAFE.compareAndSwapInt(this, stateOffset, NEW,
mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))
return false;
try { // in case call to interrupt throws exception
if (mayInterruptIfRunning) {
try {
Thread t = runner;
if (t != null)
t.interrupt();
} finally { // final state
UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED);
}
}
} finally {
finishCompletion();
}
return true;
}
/**
* @throws CancellationException {@inheritDoc}
*/
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING)
s = awaitDone(false, 0L);
return report(s);
}
/**
* @throws CancellationException {@inheritDoc}
*/
public V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException {
if (unit == null)
throw new NullPointerException();
int s = state;
if (s <= COMPLETING &&
(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
throw new TimeoutException();
return report(s);
}
/**
* Protected method invoked when this task transitions to state
* {@code isDone} (whether normally or via cancellation). The
* default implementation does nothing. Subclasses may override
* this method to invoke completion callbacks or perform
* bookkeeping. Note that you can query status inside the
* implementation of this method to determine whether this task
* has been cancelled.
*/
protected void done() { }
/**
* Sets the result of this future to the given value unless
* this future has already been set or has been cancelled.
*
* <p>This method is invoked internally by the {@link #run} method
* upon successful completion of the computation.
*
* @param v the value
*/
protected void set(V v) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = v;
UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
finishCompletion();
}
}
/**
* Causes this future to report an {@link ExecutionException}
* with the given throwable as its cause, unless this future has
* already been set or has been cancelled.
*
* <p>This method is invoked internally by the {@link #run} method
* upon failure of the computation.
*
* @param t the cause of failure
*/
protected void setException(Throwable t) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = t;
UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
finishCompletion();
}
}
public void run() {
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
null, Thread.currentThread()))
return;
try {
Callable<V> c = callable;
if (c != null && state == NEW) {
V result;
boolean ran;
try {
result = c.call();
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
if (ran)
set(result);
}
} finally {
// runner must be non-null until state is settled to
// prevent concurrent calls to run()
runner = null;
// state must be re-read after nulling runner to prevent
// leaked interrupts
int s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}
/**
* Executes the computation without setting its result, and then
* resets this future to initial state, failing to do so if the
* computation encounters an exception or is cancelled. This is
* designed for use with tasks that intrinsically execute more
* than once.
*
* @return {@code true} if successfully run and reset
*/
protected boolean runAndReset() {
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
null, Thread.currentThread()))
return false;
boolean ran = false;
int s = state;
try {
Callable<V> c = callable;
if (c != null && s == NEW) {
try {
c.call(); // don't set result
ran = true;
} catch (Throwable ex) {
setException(ex);
}
}
} finally {
// runner must be non-null until state is settled to
// prevent concurrent calls to run()
runner = null;
// state must be re-read after nulling runner to prevent
// leaked interrupts
s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
return ran && s == NEW;
}
/**
* Ensures that any interrupt from a possible cancel(true) is only
* delivered to a task while in run or runAndReset.
*/
private void handlePossibleCancellationInterrupt(int s) {
// It is possible for our interrupter to stall before getting a
// chance to interrupt us. Let's spin-wait patiently.
if (s == INTERRUPTING)
while (state == INTERRUPTING)
Thread.yield(); // wait out pending interrupt
// assert state == INTERRUPTED;
// We want to clear any interrupt we may have received from
// cancel(true). However, it is permissible to use interrupts
// as an independent mechanism for a task to communicate with
// its caller, and there is no way to clear only the
// cancellation interrupt.
//
// Thread.interrupted();
}
/**
* Simple linked list nodes to record waiting threads in a Treiber
* stack. See other classes such as Phaser and SynchronousQueue
* for more detailed explanation.
*/
static final class WaitNode {
volatile Thread thread;
volatile WaitNode next;
WaitNode() { thread = Thread.currentThread(); }
}
/**
* Removes and signals all waiting threads, invokes done(), and
* nulls out callable.
*/
private void finishCompletion() {
// assert state > COMPLETING;
for (WaitNode q; (q = waiters) != null;) {
if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
for (;;) {
Thread t = q.thread;
if (t != null) {
q.thread = null;
LockSupport.unpark(t);
}
WaitNode next = q.next;
if (next == null)
break;
q.next = null; // unlink to help gc
q = next;
}
break;
}
}
done();
callable = null; // to reduce footprint
}
/**
* Awaits completion or aborts on interrupt or timeout.
*
* @param timed true if use timed waits
* @param nanos time to wait, if timed
* @return state upon completion
*/
private int awaitDone(boolean timed, long nanos)
throws InterruptedException {
final long deadline = timed ? System.nanoTime() + nanos : 0L;
WaitNode q = null;
boolean queued = false;
for (;;) {
if (Thread.interrupted()) {
removeWaiter(q);
throw new InterruptedException();
}
int s = state;
if (s > COMPLETING) {
if (q != null)
q.thread = null;
return s;
}
else if (s == COMPLETING) // cannot time out yet
Thread.yield();
else if (q == null)
q = new WaitNode();
else if (!queued)
queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
q.next = waiters, q);
else if (timed) {
nanos = deadline - System.nanoTime();
if (nanos <= 0L) {
removeWaiter(q);
return state;
}
LockSupport.parkNanos(this, nanos);
}
else
LockSupport.park(this);
}
}
/**
* Tries to unlink a timed-out or interrupted wait node to avoid
* accumulating garbage. Internal nodes are simply unspliced
* without CAS since it is harmless if they are traversed anyway
* by releasers. To avoid effects of unsplicing from already
* removed nodes, the list is retraversed in case of an apparent
* race. This is slow when there are a lot of nodes, but we don't
* expect lists to be long enough to outweigh higher-overhead
* schemes.
*/
private void removeWaiter(WaitNode node) {
if (node != null) {
node.thread = null;
retry:
for (;;) { // restart on removeWaiter race
for (WaitNode pred = null, q = waiters, s; q != null; q = s) {
s = q.next;
if (q.thread != null)
pred = q;
else if (pred != null) {
pred.next = s;
if (pred.thread == null) // check for race
continue retry;
}
else if (!UNSAFE.compareAndSwapObject(this, waitersOffset,
q, s))
continue retry;
}
break;
}
}
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE;
private static final long stateOffset;
private static final long runnerOffset;
private static final long waitersOffset;
static {
try {
UNSAFE = sun.misc.Unsafe.getUnsafe();
Class<?> k = FutureTask.class;
stateOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("state"));
runnerOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("runner"));
waitersOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("waiters"));
} catch (Exception e) {
throw new Error(e);
}
}
}
- 使用int字段表示当前任务状态流转
- 使用链表对阻塞线程进行排队
- 提供通用的异步任务封装
执行有返回值的任务
x
public static void main(String[] args) throws Exception {
FutureTask<String> futureTask = new FutureTask<String>(new Callable<String>() {
public String call() throws Exception {
return new Task().say("Hello,World!");
}
});
new Thread(futureTask).start();
System.out.println(futureTask.isDone());//直接返回
System.out.println(Thread.currentThread().getName() + " say : " + futureTask.get());//等待结果
}
- 自动阻塞等待获取结果
- 不需要处理同步、排队
- 可以对futureTask进行操作
总结
- JDK1.0开始对多线程只提供了Thread类和Runnable接口的简单支持
- JDK的创建线程本不支持获取线程执行结果的返回值，但是通过大师一系列封装，一切都变得简单了
- 另外可以看到Runnable接口是1.0，Callable接口是1.5，那么获取异步结果在这之间JDK是有空白的
- 如果带着Callable接口回到1.0，那么Thread类的启动线程直接运行Callable.call()方法，Thread.start()方法直接返回Future，会不会更简单容易理解了
- 要是Thread.start()支持参数传递，会不会更完美了
