executor 线程池
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方法代表执行，需要传入一个Runnable，自动执行
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.
*
* 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.
*
*
This method does not wait for actively executing tasks to
* terminate. Use {@link #awaitTermination awaitTermination} to
* do that.
*
*
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 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.
*
*
* If you would like to immediately block waiting
* for a task, you can use constructions of the form
* {@code result = exec.submit(aCallable).get();}
*
*
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 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
*/
Future submit(Callable 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 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
*/
Future 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 successful 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 completed 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 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
*/
List> invokeAll(Collection> 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 completed 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 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
*/
List> invokeAll(Collection> 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 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 invokeAny(Collection> 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 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 invokeAny(Collection> tasks,
long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}
主要三个方法
void shutdown();代表关闭线程池，不能再让该线程池添加任务。但是在执行中的任务和排队中的任务不会受到影响，全部执行完才会结束
List代表结束线程池。会通过Thread.interrput()来试图结束正在执行的任务，队列中的任务不会执行，并且将其返回。也会等待所有任务都结束才会真正结束。shutdownNow();
代表执行一个带阻塞式的任务，可以使用Callable，拿到线程的返回值Future submit(Callable task);
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:
* {@code corePoolSize < 0}
* {@code keepAliveTime < 0}
* {@code maximumPoolSize <= 0}
* {@code maximumPoolSize < corePoolSize}
* @throws NullPointerException if {@code workQueue} is null
*/
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue workQueue) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), defaultHandler);
}
corePoolSize代表默认线程个数。当线程池被创建，就默认创建多少个；当线程池在一个一个回收，回收都默认个数后就不会回收了
maximumPoolSize代表最大线程格数。当线程池中线程超过最大数，就会进入排队队列中。
keepAliveTime & unit两个一起使用，代表线程等待被回收的时间。当线程执行完，具体多少后会被回收，取决与这个时间
workQueue代表任务队列。当线程池中线程超过最大数，再添加任务先添加到此队列，等待线程被回收后，线程池当前线程数小于最大数，将会取出执行。
ThreadPoolExecutor 常用实现
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue());
}
代表一个可以添加无数个任务，默认线程数0，1分钟会被回收的线程池。
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue()));
}
代表单线程的线程池
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue());
}
代表固定线程数的线程池，并且不会被回收，不能扩展。一般用来处理集中瞬时爆发的任务，之后需要调用shutdown()来结束且回收
线程同步与安全
本质就是资源问题
volatile
当子线程使用了主线程的内对象，其实子线程会把主线程的内对象拷贝到自己的内存块，当子线程这个对象的值发送改变后，主线程中这个对象不会发送变化，JVM这样做是为了效率问题。为主线程的内对象加上volatile关键字，会让此对象具有同步性，这个时候在子线程修改对象值的时候，会同步到主线程内对象上，相反主线程修改也会同步到子线程。
ATomic
使用了volatile后，对象会有同步性，但是当修改这个值是需要分几步执行时。可能在修改过程中切换到另一个线程从头开始修改，导致了程序的数据错乱。所以可以在Atomic来作为对象，比如++ --，让修改操作不仅仅具有同步性，而且具有原子性。
synchronized
同步的意思，保护代码块在多线程同时调用的时候，具有原子性，不会出现数据错误。
monitor（锁）
使用synchronized需要用到，可以理解为锁。
当为代码块使用synchronized (this) {}, 会使用当前类对象的锁。
当其他线程同时调用时，这个锁还处于未解锁，其他线程会在后面等待被执行，直至锁被解锁，再执行后面的线程任务。
当为代码块使用synchronized (${other monitor}) {}，当其他线程同时调用时，执行的方法中有这个锁才会等待执行，否则不受锁限制。
死锁
当A方法，代码块用了A锁，A锁中使用了B锁；
当B方法，代码块用了B锁，B锁中使用了A锁；
A线程调用A方法，B线程调用B方法，且同时调用。A执行后A锁会锁住，B执行后B锁会锁住。A锁中执行到B锁的时候，发现B锁锁住了，就会等待解锁；B锁中执行到A锁的时候，发现A锁锁住了，也会等待解锁。这样就成死锁了。
乐观锁悲观锁
这是操作数据库才能的遇到的问题，一般在Android中不会出现
乐观锁的意思就是不加锁，从数据库拿了数据做业务逻辑修改数据，存入数据库的时候，再拿数据库最新数据和当时拿出来的数据对比，值没有被其他人修改就提价修改；值有差别，则再取出值重新做业务逻辑。
悲观锁的意思就是我从数据库开始读取数据的时候，我就加锁，不允许其他人修改数据。我拿到数据处理完直接更新数据库，之后我再解锁，让别人操作。
静态锁
一般用于单例模式
getInstance() 可能存在多线程同时调用，防止多次初始化并且覆盖操作，需要加上静态锁
synchronized(Class.class)