executor 線程池

public interface Executor {
    /**
     * Executes the given command at some time in the future.  The command
     * may execute in a new thread, in a pooled thread, or in the calling
     * thread, at the discretion of the {@code Executor} implementation.
     *
     * @param command the runnable task
     * @throws RejectedExecutionException if this task cannot be
     * accepted for execution
     * @throws NullPointerException if command is null
     */
    void execute(Runnable command);
}

execute方法代表執(zhí)行，需要傳入一個Runnable，自動執(zhí)行

public interface ExecutorService extends Executor {

    /**
     * Initiates an orderly shutdown in which previously submitted
     * tasks are executed, but no new tasks will be accepted.
     * Invocation has no additional effect if already shut down.
     *
     * <p>This method does not wait for previously submitted tasks to
     * complete execution.  Use {@link #awaitTermination awaitTermination}
     * to do that.
     */
    void shutdown();

    /**
     * Attempts to stop all actively executing tasks, halts the
     * processing of waiting tasks, and returns a list of the tasks
     * that were awaiting execution.
     *
     * <p>This method does not wait for actively executing tasks to
     * terminate.  Use {@link #awaitTermination awaitTermination} to
     * do that.
     *
     * <p>There are no guarantees beyond best-effort attempts to stop
     * processing actively executing tasks.  For example, typical
     * implementations will cancel via {@link Thread#interrupt}, so any
     * task that fails to respond to interrupts may never terminate.
     *
     * @return list of tasks that never commenced execution
     */
    List<Runnable> shutdownNow();

    /**
     * Returns {@code true} if this executor has been shut down.
     *
     * @return {@code true} if this executor has been shut down
     */
    boolean isShutdown();

    /**
     * Returns {@code true} if all tasks have completed following shut down.
     * Note that {@code isTerminated} is never {@code true} unless
     * either {@code shutdown} or {@code shutdownNow} was called first.
     *
     * @return {@code true} if all tasks have completed following shut down
     */
    boolean isTerminated();

    /**
     * Blocks until all tasks have completed execution after a shutdown
     * request, or the timeout occurs, or the current thread is
     * interrupted, whichever happens first.
     *
     * @param timeout the maximum time to wait
     * @param unit the time unit of the timeout argument
     * @return {@code true} if this executor terminated and
     *         {@code false} if the timeout elapsed before termination
     * @throws InterruptedException if interrupted while waiting
     */
    boolean awaitTermination(long timeout, TimeUnit unit)
        throws InterruptedException;

    /**
     * Submits a value-returning task for execution and returns a
     * Future representing the pending results of the task. The
     * Future's {@code get} method will return the task's result upon
     * successful completion.
     *
     * <p>
     * If you would like to immediately block waiting
     * for a task, you can use constructions of the form
     * {@code result = exec.submit(aCallable).get();}
     *
     * <p>Note: The {@link Executors} class includes a set of methods
     * that can convert some other common closure-like objects,
     * for example, {@link java.security.PrivilegedAction} to
     * {@link Callable} form so they can be submitted.
     *
     * @param task the task to submit
     * @param <T> the type of the task's result
     * @return a Future representing pending completion of the task
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     * @throws NullPointerException if the task is null
     */
    <T> Future<T> submit(Callable<T> task);

    /**
     * Submits a Runnable task for execution and returns a Future
     * representing that task. The Future's {@code get} method will
     * return the given result upon successful completion.
     *
     * @param task the task to submit
     * @param result the result to return
     * @param <T> the type of the result
     * @return a Future representing pending completion of the task
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     * @throws NullPointerException if the task is null
     */
    <T> Future<T> submit(Runnable task, T result);

    /**
     * Submits a Runnable task for execution and returns a Future
     * representing that task. The Future's {@code get} method will
     * return {@code null} upon <em>successful</em> completion.
     *
     * @param task the task to submit
     * @return a Future representing pending completion of the task
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     * @throws NullPointerException if the task is null
     */
    Future<?> submit(Runnable task);

    /**
     * Executes the given tasks, returning a list of Futures holding
     * their status and results when all complete.
     * {@link Future#isDone} is {@code true} for each
     * element of the returned list.
     * Note that a <em>completed</em> task could have
     * terminated either normally or by throwing an exception.
     * The results of this method are undefined if the given
     * collection is modified while this operation is in progress.
     *
     * @param tasks the collection of tasks
     * @param <T> the type of the values returned from the tasks
     * @return a list of Futures representing the tasks, in the same
     *         sequential order as produced by the iterator for the
     *         given task list, each of which has completed
     * @throws InterruptedException if interrupted while waiting, in
     *         which case unfinished tasks are cancelled
     * @throws NullPointerException if tasks or any of its elements are {@code null}
     * @throws RejectedExecutionException if any task cannot be
     *         scheduled for execution
     */
    <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
        throws InterruptedException;

    /**
     * Executes the given tasks, returning a list of Futures holding
     * their status and results
     * when all complete or the timeout expires, whichever happens first.
     * {@link Future#isDone} is {@code true} for each
     * element of the returned list.
     * Upon return, tasks that have not completed are cancelled.
     * Note that a <em>completed</em> task could have
     * terminated either normally or by throwing an exception.
     * The results of this method are undefined if the given
     * collection is modified while this operation is in progress.
     *
     * @param tasks the collection of tasks
     * @param timeout the maximum time to wait
     * @param unit the time unit of the timeout argument
     * @param <T> the type of the values returned from the tasks
     * @return a list of Futures representing the tasks, in the same
     *         sequential order as produced by the iterator for the
     *         given task list. If the operation did not time out,
     *         each task will have completed. If it did time out, some
     *         of these tasks will not have completed.
     * @throws InterruptedException if interrupted while waiting, in
     *         which case unfinished tasks are cancelled
     * @throws NullPointerException if tasks, any of its elements, or
     *         unit are {@code null}
     * @throws RejectedExecutionException if any task cannot be scheduled
     *         for execution
     */
    <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks,
                                  long timeout, TimeUnit unit)
        throws InterruptedException;

    /**
     * Executes the given tasks, returning the result
     * of one that has completed successfully (i.e., without throwing
     * an exception), if any do. Upon normal or exceptional return,
     * tasks that have not completed are cancelled.
     * The results of this method are undefined if the given
     * collection is modified while this operation is in progress.
     *
     * @param tasks the collection of tasks
     * @param <T> the type of the values returned from the tasks
     * @return the result returned by one of the tasks
     * @throws InterruptedException if interrupted while waiting
     * @throws NullPointerException if tasks or any element task
     *         subject to execution is {@code null}
     * @throws IllegalArgumentException if tasks is empty
     * @throws ExecutionException if no task successfully completes
     * @throws RejectedExecutionException if tasks cannot be scheduled
     *         for execution
     */
    <T> T invokeAny(Collection<? extends Callable<T>> tasks)
        throws InterruptedException, ExecutionException;

    /**
     * Executes the given tasks, returning the result
     * of one that has completed successfully (i.e., without throwing
     * an exception), if any do before the given timeout elapses.
     * Upon normal or exceptional return, tasks that have not
     * completed are cancelled.
     * The results of this method are undefined if the given
     * collection is modified while this operation is in progress.
     *
     * @param tasks the collection of tasks
     * @param timeout the maximum time to wait
     * @param unit the time unit of the timeout argument
     * @param <T> the type of the values returned from the tasks
     * @return the result returned by one of the tasks
     * @throws InterruptedException if interrupted while waiting
     * @throws NullPointerException if tasks, or unit, or any element
     *         task subject to execution is {@code null}
     * @throws TimeoutException if the given timeout elapses before
     *         any task successfully completes
     * @throws ExecutionException if no task successfully completes
     * @throws RejectedExecutionException if tasks cannot be scheduled
     *         for execution
     */
    <T> T invokeAny(Collection<? extends Callable<T>> tasks,
                    long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException;
}

主要三個方法
void shutdown(); 代表關閉線程池，不能再讓該線程池添加任務。但是在執(zhí)行中的任務和排隊中的任務不會受到影響，全部執(zhí)行完才會結(jié)束
List<Runnable> shutdownNow(); 代表結(jié)束線程池。會通過Thread.interrput()來試圖結(jié)束正在執(zhí)行的任務，隊列中的任務不會執(zhí)行，并且將其返回。也會等待所有任務都結(jié)束才會真正結(jié)束。
<T> Future<T> submit(Callable<T> task); 代表執(zhí)行一個帶阻塞式的任務，可以使用Callable，拿到線程的返回值

ThreadPoolExecutor

/**
 * Creates a new {@code ThreadPoolExecutor} with the given initial
 * parameters and default thread factory and rejected execution handler.
 * It may be more convenient to use one of the {@link Executors} factory
 * methods instead of this general purpose constructor.
 *
 * @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.
 * @throws IllegalArgumentException if one of the following holds:<br>
 *         {@code corePoolSize < 0}<br>
 *         {@code keepAliveTime < 0}<br>
 *         {@code maximumPoolSize <= 0}<br>
 *         {@code maximumPoolSize < corePoolSize}
 * @throws NullPointerException if {@code workQueue} is null
 */
public ThreadPoolExecutor(int corePoolSize,
                          int maximumPoolSize,
                          long keepAliveTime,
                          TimeUnit unit,
                          BlockingQueue<Runnable> workQueue) {
    this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
         Executors.defaultThreadFactory(), defaultHandler);
}

corePoolSize 代表默認線程個數(shù)。當線程池被創(chuàng)建，就默認創(chuàng)建多少個；當線程池在一個一個回收，回收都默認個數(shù)后就不會回收了
maximumPoolSize 代表最大線程格數(shù)。當線程池中線程超過最大數(shù)，就會進入排隊隊列中。
keepAliveTime & unit 兩個一起使用，代表線程等待被回收的時間。當線程執(zhí)行完，具體多少后會被回收，取決與這個時間
workQueue 代表任務隊列。當線程池中線程超過最大數(shù)，再添加任務先添加到此隊列，等待線程被回收后，線程池當前線程數(shù)小于最大數(shù)，將會取出執(zhí)行。

ThreadPoolExecutor 常用實現(xiàn)

public static ExecutorService newCachedThreadPool() {
    return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                  60L, TimeUnit.SECONDS,
                                  new SynchronousQueue<Runnable>());
}

代表一個可以添加無數(shù)個任務，默認線程數(shù)0，1分鐘會被回收的線程池。

public static ExecutorService newSingleThreadExecutor() {
    return new FinalizableDelegatedExecutorService
        (new ThreadPoolExecutor(1, 1,
                                0L, TimeUnit.MILLISECONDS,
                                new LinkedBlockingQueue<Runnable>()));
}

代表單線程的線程池

public static ExecutorService newFixedThreadPool(int nThreads) {
    return new ThreadPoolExecutor(nThreads, nThreads,
                                  0L, TimeUnit.MILLISECONDS,
                                  new LinkedBlockingQueue<Runnable>());
}

代表固定線程數(shù)的線程池，并且不會被回收，不能擴展。一般用來處理集中瞬時爆發(fā)的任務，之后需要調(diào)用shutdown()來結(jié)束且回收

線程同步與安全

本質(zhì)就是資源問題

volatile

當子線程使用了主線程的內(nèi)對象，其實子線程會把主線程的內(nèi)對象拷貝到自己的內(nèi)存塊，當子線程這個對象的值發(fā)送改變后，主線程中這個對象不會發(fā)送變化，JVM這樣做是為了效率問題。為主線程的內(nèi)對象加上volatile關鍵字，會讓此對象具有同步性，這個時候在子線程修改對象值的時候，會同步到主線程內(nèi)對象上，相反主線程修改也會同步到子線程。

ATomic

使用了volatile后，對象會有同步性，但是當修改這個值是需要分幾步執(zhí)行時?？赡茉谛薷倪^程中切換到另一個線程從頭開始修改，導致了程序的數(shù)據(jù)錯亂。所以可以在Atomic來作為對象，比如++ --，讓修改操作不僅僅具有同步性，而且具有原子性。

synchronized

同步的意思，保護代碼塊在多線程同時調(diào)用的時候，具有原子性，不會出現(xiàn)數(shù)據(jù)錯誤。

monitor（鎖）

使用synchronized需要用到，可以理解為鎖。
當為代碼塊使用synchronized (this) {}, 會使用當前類對象的鎖。
當其他線程同時調(diào)用時，這個鎖還處于未解鎖，其他線程會在后面等待被執(zhí)行，直至鎖被解鎖，再執(zhí)行后面的線程任務。
當為代碼塊使用synchronized (${other monitor}) {}，當其他線程同時調(diào)用時，執(zhí)行的方法中有這個鎖才會等待執(zhí)行，否則不受鎖限制。

死鎖

當A方法，代碼塊用了A鎖，A鎖中使用了B鎖；
當B方法，代碼塊用了B鎖，B鎖中使用了A鎖；
A線程調(diào)用A方法，B線程調(diào)用B方法，且同時調(diào)用。A執(zhí)行后A鎖會鎖住，B執(zhí)行后B鎖會鎖住。A鎖中執(zhí)行到B鎖的時候，發(fā)現(xiàn)B鎖鎖住了，就會等待解鎖；B鎖中執(zhí)行到A鎖的時候，發(fā)現(xiàn)A鎖鎖住了，也會等待解鎖。這樣就成死鎖了。

樂觀鎖悲觀鎖

這是操作數(shù)據(jù)庫才能的遇到的問題，一般在Android中不會出現(xiàn)
樂觀鎖的意思就是不加鎖，從數(shù)據(jù)庫拿了數(shù)據(jù)做業(yè)務邏輯修改數(shù)據(jù)，存入數(shù)據(jù)庫的時候，再拿數(shù)據(jù)庫最新數(shù)據(jù)和當時拿出來的數(shù)據(jù)對比，值沒有被其他人修改就提價修改；值有差別，則再取出值重新做業(yè)務邏輯。
悲觀鎖的意思就是我從數(shù)據(jù)庫開始讀取數(shù)據(jù)的時候，我就加鎖，不允許其他人修改數(shù)據(jù)。我拿到數(shù)據(jù)處理完直接更新數(shù)據(jù)庫，之后我再解鎖，讓別人操作。

靜態(tài)鎖

一般用于單例模式
getInstance() 可能存在多線程同時調(diào)用，防止多次初始化并且覆蓋操作，需要加上靜態(tài)鎖
synchronized(Class.class)