细数Android开源项目中那些频繁使用的并发库中的类

这篇blog旨在帮助大家 梳理一下前面分析的那些开源代码中喜欢使用的一些类，这对我们真正理解这些项目是有极大好处的，以后遇到类似问题 我们就可以自己模仿他们也写

出类似的代码。

1.ExecutorService

这个类实际上就是一个接口

public interface ExecutorService extends Executor {

我们可以看看有哪些频繁使用的类 是实现了这个接口的，其实主要就是3个。

 /**
     * Creates a thread pool that reuses a fixed number of threads
     * operating off a shared unbounded queue.  At any point, at most
     * {@code nThreads} threads will be active processing tasks.
     * If additional tasks are submitted when all threads are active,
     * they will wait in the queue until a thread is available.
     * If any thread terminates due to a failure during execution
     * prior to shutdown, a new one will take its place if needed to
     * execute subsequent tasks.  The threads in the pool will exist
     * until it is explicitly {@link ExecutorService#shutdown shutdown}.
     *
     * @param nThreads the number of threads in the pool
     * @return the newly created thread pool
     * @throws IllegalArgumentException if {@code nThreads <= 0}
     */
    public static ExecutorService newFixedThreadPool(int nThreads) {
        return new ThreadPoolExecutor(nThreads, nThreads,
                                      0L, TimeUnit.MILLISECONDS,
                                      new LinkedBlockingQueue<Runnable>());
    }

这个线程池，就是有固定线程数的一个线程池，有共享的无界队列来运行这些线程。

/**
     * Creates a thread pool that creates new threads as needed, but
     * will reuse previously constructed threads when they are
     * available.  These pools will typically improve the performance
     * of programs that execute many short-lived asynchronous tasks.
     * Calls to {@code execute} will reuse previously constructed
     * threads if available. If no existing thread is available, a new
     * thread will be created and added to the pool. Threads that have
     * not been used for sixty seconds are terminated and removed from
     * the cache. Thus, a pool that remains idle for long enough will
     * not consume any resources. Note that pools with similar
     * properties but different details (for example, timeout parameters)
     * may be created using {@link ThreadPoolExecutor} constructors.
     *
     * @return the newly created thread pool
     */
    public static ExecutorService newCachedThreadPool() {
        return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                      60L, TimeUnit.SECONDS,
                                      new SynchronousQueue<Runnable>());
    }

这个线程池，是根据需要来创建这些线程的，但是以前构造过的线程 必要时可以重用他们，所以这个在很多android的开源项目里都有用到，很频繁，对于执行很多短期的异步任务来说，这个线程池可以极大的提高程序的性能。

/**
     * Creates an Executor that uses a single worker thread operating
     * off an unbounded queue. (Note however that if this single
     * thread terminates due to a failure during execution prior to
     * shutdown, a new one will take its place if needed to execute
     * subsequent tasks.)  Tasks are guaranteed to execute
     * sequentially, and no more than one task will be active at any
     * given time. Unlike the otherwise equivalent
     * {@code newFixedThreadPool(1)} the returned executor is
     * guaranteed not to be reconfigurable to use additional threads.
     *
     * @return the newly created single-threaded Executor
     */
    public static ExecutorService newSingleThreadExecutor() {
        return new FinalizableDelegatedExecutorService
            (new ThreadPoolExecutor(1, 1,
                                    0L, TimeUnit.MILLISECONDS,
                                    new LinkedBlockingQueue<Runnable>()));
    }

而这个线程池就比较特殊一点，他只有一个worker线程在工作。

来看第一个程序：

public class Test1 {

    public static void main(String[] args) {
        ExecutorService exectrorService = Executors.newFixedThreadPool(10);
        // execute异步的方法去执行这个runnable 但是这种方法无法取得运行之后的返回值
        exectrorService.execute(new Runnable() {
            @Override
            public void run() {
                // TODO Auto-generated method stub
                int i = 0;
                while (true) {
                    try {
                        Thread.sleep(2000);
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                    System.out.println(i);
                    i++;
                }
            }

        });

        exectrorService.execute(new Runnable() {
            @Override
            public void run() {
                // TODO Auto-generated method stub
                int i = 100;
                while (true) {
                    try {
                        Thread.sleep(2000);
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                    System.out.println(i);
                    i++;
                }
            }

        });

很简单 没有什么好说的只是为了演示一下这个方法，继续往下看：

public class Test1 {

    public static void main(String[] args) {
        ExecutorService exectrorService = Executors.newFixedThreadPool(10);
        Future future = exectrorService.submit(new Runnable() {

            @Override
            public void run() {
                System.out.println("thread start");
                // TODO Auto-generated method stub
                try {
                    Thread.sleep(13000);
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
                System.out.println("task done");
            }
        });
        System.out.println("ready to print status");
        try {
            // 执行完毕以后才会返回null,如果线程还没有执行完毕 那这个地方会阻塞
            System.out.println("future.get ==" + future.get());
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (ExecutionException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        System.out.println("finish ready");

这个就是为了演示get方法是个阻塞方法的 我们可以看下打印的日志。

程序一开始运行 日志如下：

thread start
ready to print status

当线程执行完毕大约过了13秒以后

才会继续输入日志如下：

task done
future.get ==null
finish ready

继续看下面的例子：

package com.android.testclass;

import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

public class Test1 {

    public static void main(String[] args) {
        ExecutorService exectrorService = Executors.newFixedThreadPool(10);
        // 这个submit方法则会保证结束以后把结果返回给future，用泛型定义的方法 你可以
        // 用任意的object代替T
        Future future = exectrorService.submit(new Callable<String>() {
            @Override
            public String call() throws Exception {
                // TODO Auto-generated method stub
                System.out.println("call start");

                Thread.sleep(5000);

                return "call done";
            }
        });
        System.out.println("ready to print");
        try {
            System.out.println("future.get()" + future.get());
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (ExecutionException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        System.out.println("finish");

    }
}

同样是submit方法 只不过这次我们换了一个参数 这次是callable参数，这么做的好处就是执行完毕以后可以拿到结果了

一开始输出：

call start
ready to print

线程执行完毕以后输出：

future.get()call done
finish

然后我们继续看invokeany这个函数：

package com.android.testclass;

import java.util.HashSet;
import java.util.Set;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class Test2 {

    public static void main(String[] args) {
        ExecutorService executorService = Executors.newFixedThreadPool(10);
        Set<Callable<String>> callables = new HashSet<Callable<String>>();
        callables.add(new Callable<String>() {
            @Override
            public String call() throws Exception {
                // TODO Auto-generated method stub
                System.out.println("task 1 start");
                Thread.sleep(3000);
                return "Task 1";
            }
        });
        callables.add(new Callable<String>() {
            @Override
            public String call() throws Exception {
                System.out.println("task 2 start");
                Thread.sleep(3000);
                return "Task 2";
            }
        });
        callables.add(new Callable<String>() {
            @Override
            public String call() throws Exception {
                System.out.println("task 3 start");
                Thread.sleep(3000);
                return "Task 3";
            }
        });
        System.out.println("ready to print");
        try {
            //返回某一个callable执行结束的结果，结果并不确定
            String result = executorService.invokeAny(callables);
            System.out.println("result==" + result);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (ExecutionException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        System.out.println("done to print");

    }
}

输出我就不放了 大家可以自己跑一下。这个函数用的比较少。

那下面这个invokeall函数用的就比较多了

package com.android.testclass;

import java.util.HashSet;
import java.util.List;
import java.util.Set;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

public class Test3 {

    public static void main(String[] args) {
        ExecutorService executorService = Executors.newFixedThreadPool(10);
        Set<Callable<String>> callables = new HashSet<Callable<String>>();
        callables.add(new Callable<String>() {
            @Override
            public String call() throws Exception {
                // TODO Auto-generated method stub
                System.out.println("task 1 start");
                Thread.sleep(3000);
                return "Task 1";
            }
        });
        callables.add(new Callable<String>() {
            @Override
            public String call() throws Exception {
                System.out.println("task 2 start");
                Thread.sleep(6000);
                return "Task 2";
            }
        });
        callables.add(new Callable<String>() {
            @Override
            public String call() throws Exception {
                System.out.println("task 3 start");
                Thread.sleep(9000);
                return "Task 3";
            }
        });
        System.out.println("ready to print");

        try {
            // invoke方法也是阻塞方法，一定是所有callable都执行完毕才会返回结果
            List<Future<String>> futures = executorService.invokeAll(callables);
            System.out.println("invoke done");
            for (Future<String> future : futures) {
                System.out.println("future.get=" + future.get());
                System.out.println("get done");
            }
            System.out.println("all get done");
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (ExecutionException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

    }
}

总的来说，在android里如果你要使用线程池的话，那上面的这些方法 基本就肯定足够你使用了。

2.ConcurrentHashMap

这个类，相信很多人都不陌生，我就略微提一下，很多人以前在单线程的时候使用hashmap，多线程的时候使用hashtable，这么做虽然是对的，

但是hashtable里的源码说明了 这是直接对整个map进行加锁，效率是很低的，而这个concurrenthashmap的读操作几乎不会有锁，

而写操作由于采用了分段处理，所以写操作的锁 的概率和次数也大大降低。总体来说这是一个效率极高的 可适用于并发性的hashmap。

例子和原理 网上有很多 我这里就不放了。

此外和他类似的还有LinkedHashMap，实现LRU的最好选择，这个也不多讲，只是提一下，网上资料很多。

3.PriorityBlockingQueue

这个就是优先级队列，当然也是支持并发的，这个队列里存放的对象 必须是实现了Comparable 接口的。并且小的是在这个队列前面的 大的就一定是在队列的后面。

比如说我们先定义一个类：

package com.android.testclass;

public class PriorityEntity implements Comparable<PriorityEntity> {

    private static int count = 0;
    private int id = count++;
    private int priority;
    private int index = 0;

    public PriorityEntity(int priority, int index) {
        // TODO Auto-generated constructor stub
        this.priority = priority;
        this.index = index;
    }

    @Override
    public String toString() {
        return "PriorityEntity [id=" + id + ", priority=" + priority + ", index=" + index + "]";
    }

    @Override
    public int compareTo(PriorityEntity o) {
        // TODO Auto-generated method stub
        return this.priority > o.priority ? 1 : this.priority < o.priority ? -1 : 0;
    }

}

那个静态变量就表示索引的，构造出一个对象 索引就加1. 然后我们来写一下测试这个队列的代码：

package com.android.testclass;

import java.util.Random;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.PriorityBlockingQueue;
import java.util.concurrent.TimeUnit;

public class Test6 {

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        PriorityBlockingQueue q = new PriorityBlockingQueue<>();
        Random r = new Random(47);
        ExecutorService se = Executors.newCachedThreadPool();
        //往队列里 放对象,priority的值是 随即的
        se.execute(new Runnable() {

            @Override
            public void run() {
                // TODO Auto-generated method stub
                int i = 0;
                while (true) {
                    q.put(new PriorityEntity(r.nextInt(10), i++));

                    try {
                        TimeUnit.MILLISECONDS.sleep(r.nextInt(1000));
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }

                }
            }
        });
        //从队列里 取对象,然后把队列里剩余的值打出来 就会发现 每次取出来的都是最小的那个 剩下的都是从小到大排序好的
        se.execute(new Runnable() {

            @Override
            public void run() {
                // TODO Auto-generated method stub
                while (true) {
                    try {
                        System.out.println(("take-- " + q.take() + " left:-- [" + q.toString() + "]"));
                    } catch (InterruptedException e1) {
                        // TODO Auto-generated catch block
                        e1.printStackTrace();
                    }
                    try {
                        TimeUnit.MILLISECONDS.sleep(r.nextInt(1000));
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }

                }
            }
        });

    }

}

截取一段日志 可以得到我们注释里的结论：

take-- PriorityEntity  [priority=8, index=0] left:-- [[]]
take-- PriorityEntity  [priority=1, index=1] left:-- [[]]
take-- PriorityEntity  [priority=8, index=2] left:-- [[]]
take-- PriorityEntity  [priority=7, index=3] left:-- [[PriorityEntity  [priority=8, index=4]]]
take-- PriorityEntity  [priority=8, index=4] left:-- [[PriorityEntity  [priority=9, index=5]]]
take-- PriorityEntity  [priority=1, index=6] left:-- [[PriorityEntity  [priority=8, index=7], PriorityEntity  [priority=9, index=5]]]
take-- PriorityEntity  [priority=8, index=7] left:-- [[PriorityEntity  [priority=9, index=5]]]
take-- PriorityEntity  [priority=2, index=8] left:-- [[PriorityEntity  [priority=9, index=5]]]
take-- PriorityEntity  [priority=9, index=5] left:-- [[]]
take-- PriorityEntity  [priority=5, index=9] left:-- [[]]
take-- PriorityEntity  [priority=4, index=10] left:-- [[]]
take-- PriorityEntity  [priority=4, index=13] left:-- [[PriorityEntity  [priority=6, index=11], PriorityEntity  [priority=6, index=12]]]
take-- PriorityEntity  [priority=3, index=14] left:-- [[PriorityEntity  [priority=6, index=16], PriorityEntity  [priority=6, index=12], PriorityEntity  [priority=6, index=11], PriorityEntity  [priority=8, index=15]]]
take-- PriorityEntity  [priority=6, index=16] left:-- [[PriorityEntity  [priority=6, index=12], PriorityEntity  [priority=8, index=15], PriorityEntity  [priority=6, index=11]]]
take-- PriorityEntity  [priority=6, index=12] left:-- [[PriorityEntity  [priority=6, index=17], PriorityEntity  [priority=8, index=15], PriorityEntity  [priority=6, index=11]]]
take-- PriorityEntity  [priority=6, index=17] left:-- [[PriorityEntity  [priority=6, index=11], PriorityEntity  [priority=8, index=15], PriorityEntity  [priority=8, index=18]]]
take-- PriorityEntity  [priority=6, index=11] left:-- [[PriorityEntity  [priority=8, index=18], PriorityEntity  [priority=8, index=15]]]
take-- PriorityEntity  [priority=4, index=19] left:-- [[PriorityEntity  [priority=8, index=18], PriorityEntity  [priority=8, index=15]]]
take-- PriorityEntity  [priority=8, index=18] left:-- [[PriorityEntity  [priority=8, index=15]]]
take-- PriorityEntity  [priority=7, index=20] left:-- [[PriorityEntity  [priority=8, index=15]]]
take-- PriorityEntity  [priority=2, index=21] left:-- [[PriorityEntity  [priority=4, index=22], PriorityEntity  [priority=8, index=15]]]
take-- PriorityEntity  [priority=4, index=22] left:-- [[PriorityEntity  [priority=8, index=23], PriorityEntity  [priority=8, index=15]]]
take-- PriorityEntity  [priority=8, index=23] left:-- [[PriorityEntity  [priority=8, index=15]]]
take-- PriorityEntity  [priority=5, index=24] left:-- [[PriorityEntity  [priority=8, index=15]]]
take-- PriorityEntity  [priority=2, index=25] left:-- [[PriorityEntity  [priority=8, index=26], PriorityEntity  [priority=8, index=15]]]
take-- PriorityEntity  [priority=3, index=27] left:-- [[PriorityEntity  [priority=4, index=28], PriorityEntity  [priority=8, index=15], PriorityEntity  [priority=8, index=26]]]
take-- PriorityEntity  [priority=1, index=30] left:-- [[PriorityEntity  [priority=4, index=28], PriorityEntity  [priority=7, index=29], PriorityEntity  [priority=8, index=26], PriorityEntity  [priority=8, index=15], PriorityEntity  [priority=8, index=31]]]
take-- PriorityEntity  [priority=4, index=28] left:-- [[PriorityEntity  [priority=7, index=29], PriorityEntity  [priority=8, index=15], PriorityEntity  [priority=8, index=26], PriorityEntity  [priority=9, index=32], PriorityEntity  [priority=8, index=31]]]

有兴趣的话可以看看java里面 有几种类 都实现了AbstractQueue,可以挑选出适合自己业务里的队列，减少开发难度

public abstract class AbstractQueue<E>
    extends AbstractCollection<E>
    implements Queue<E> {

4.CopyOnWriteArrayList

考虑这样一种场景，一个list，被好几个线程同时读写，那一般都会报错。

Exception in thread "pool-1-thread-7" java.util.ConcurrentModificationException
    at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
    at java.util.ArrayList$Itr.next(Unknown Source)
    at com.android.testclass.Test7$ReadTask.run(Test7.java:35)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)
Exception in thread "pool-1-thread-6" java.util.ConcurrentModificationException
    at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
    at java.util.ArrayList$Itr.next(Unknown Source)
    at com.android.testclass.Test7$ReadTask.run(Test7.java:35)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)

于是很多人就喜欢用Collections.synchronizedList() 来处理，但是这样做在很多时候效率是低的，比如

假设现在告诉你，你需要设计一个缓存list，你就应该使用CopyOnWrite这个类了，因为缓存大家都知道，读操作比较多，而写操作除了在初始建立缓存的阶段，其他时候很少使用。

他的原理也很简单，就是你在用迭代器写操作的时候 是把原来的数据拷贝了一份镜像在内存中，而你在读的时候 是读的本体，写操作写完以后才会覆盖掉原来的本地。所以可以

得知 这个类对于频繁读的同步性list 是非常有效的。使用方法也很简单。

        List<String> list = new CopyOnWriteArrayList<String>();

5.ThreadLocal

这个类也是很有效，很多开源作者喜欢用的一个类，他主要的作用是为每个线程创造一个变量的副本互相不会影响。很多人不理解这句话，

对于多线程操作来说 分为两种

1 第一种，线程和线程之间互相读取操作，比如全局的计数器这种，a线程要加，b线程也要加，每次加的时候 都要读取最新的计数器的状态。这是最常见的一种同步操作。

2 第二种，session，session一个用户一个，互相不影响，大家维持自己的就可以，他的目标就是a的seesion a自己操作 保存 读取，b的seesion也是自己维护，和其他人无关。

换一句话说 如果你需要多个线程之间通信，那就用同步机制，

如果你不需要线程与线程之间通信，只要互相别影响 不让他们发生冲突 则threadlocal是最佳选择。

package com.android.testclass;

public class Test8 {

    static final ThreadLocal<Integer> local = new ThreadLocal<Integer>() {

        protected Integer initialValue() {

            return 0;
        };

    };

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        Thread[] threads = new Thread[5];
        for (int i = 0; i < 5; i++) {
            threads[i] = new Thread(new Runnable() {

                @Override
                public void run() {
                    // TODO Auto-generated method stub

                    int num = local.get();
                    for (int i = 0; i < 5; i++) {
                        num++;
                    }
                    local.set(num);
                    System.out.println(Thread.currentThread().getName() + " : " + local.get());

                }
            }, "thread-" + i);
        }

        for (Thread thread : threads) {
            thread.start();
        }

    }

}

看下输出

thread-0 : 5
thread-4 : 5
thread-1 : 5
thread-3 : 5
thread-2 : 5

接着看下面的

package com.android.testclass;

public class Test9 {

    private static Index num = new Index();
    // 创建一个Index类型的本地变量
    private static ThreadLocal<Index> local = new ThreadLocal<Index>() {
        @Override
        protected Index initialValue() {
            return num;
        }
    };

    public static void main(String[] args) throws InterruptedException {
        Thread[] threads = new Thread[5];
        for (int j = 0; j < 5; j++) {
            threads[j] = new Thread(new Runnable() {
                @Override
                public void run() {
                    // 取出当前线程的本地变量，并累加1000次
                    Index index = local.get();
                    for (int i = 0; i < 1000; i++) {
                        index.increase();
                    }
                    System.out.println(Thread.currentThread().getName() + " : " + index.num);

                }
            }, "Thread-" + j);
        }
        for (Thread thread : threads) {
            thread.start();
        }
    }

    static class Index {
        int num;

        public void increase() {
            num++;
        }
    }

}

看输出

Thread-1 : 2594
Thread-4 : 3594
Thread-2 : 2594
Thread-0 : 2594
Thread-3 : 4594

是因为第10行，那边放的是一个静态变量的引用，所以输出的结果不是我们想象的

其实只要改成

private static ThreadLocal<Index> local = new ThreadLocal<Index>() {
        @Override
        protected Index initialValue() {
            return new Index();
        }
    };

结果就是正确的：

Thread-2 : 1000
Thread-3 : 1000
Thread-0 : 1000
Thread-4 : 1000
Thread-1 : 1000