Android中的Semaphore

信号量，了解过操作系统的人都知道，信号量是用来做什么的···

在Android中，已经提供了Semaphore来帮助我们使用~

那么，在开发中这家伙有什么用呢？

用的地方不多，但是却真的是好用至极！

#0 举个栗子

我相信很多人在开发中都会遇到这样的事情：当要对一个资源进行多次数据读取并且修改操作时，会遇到执行的速度快慢不一致导致修改值发生改变的情况。

比如如下代码：

　　 private void Start() {
        for (int i = 10; i >= 0; i--) {
            LogString = "num:" + i;
            printLogString(i);
        }
    }

    private void printLogString(final int delayTime) {
        Log.e("TAG", "Runnable" + delayTime);
        new Thread(new Runnable() {
            @Override
            public void run() {
                Log.e("TAG", LogString);
            }
        }).start();
    }

打印出的日志会不一致，如：

04-03 15:33:07.294 17078-17078/com.gao.luna.semaphoretest E/TAG: Runnable10
04-03 15:33:07.296 17078-17078/com.gao.luna.semaphoretest E/TAG: Runnable9
04-03 15:33:07.296 17078-17777/com.gao.luna.semaphoretest E/TAG: num:9
04-03 15:33:07.297 17078-17078/com.gao.luna.semaphoretest E/TAG: Runnable8
04-03 15:33:07.297 17078-17078/com.gao.luna.semaphoretest E/TAG: Runnable7
04-03 15:33:07.298 17078-17778/com.gao.luna.semaphoretest E/TAG: num:7
04-03 15:33:07.299 17078-17078/com.gao.luna.semaphoretest E/TAG: Runnable6
04-03 15:33:07.299 17078-17779/com.gao.luna.semaphoretest E/TAG: num:6
04-03 15:33:07.300 17078-17078/com.gao.luna.semaphoretest E/TAG: Runnable5
04-03 15:33:07.301 17078-17780/com.gao.luna.semaphoretest E/TAG: num:5
04-03 15:33:07.301 17078-17078/com.gao.luna.semaphoretest E/TAG: Runnable4
04-03 15:33:07.302 17078-17781/com.gao.luna.semaphoretest E/TAG: num:4
04-03 15:33:07.303 17078-17078/com.gao.luna.semaphoretest E/TAG: Runnable3
04-03 15:33:07.303 17078-17782/com.gao.luna.semaphoretest E/TAG: num:3
04-03 15:33:07.304 17078-17783/com.gao.luna.semaphoretest E/TAG: num:3
04-03 15:33:07.305 17078-17078/com.gao.luna.semaphoretest E/TAG: Runnable2
04-03 15:33:07.305 17078-17784/com.gao.luna.semaphoretest E/TAG: num:2
04-03 15:33:07.305 17078-17078/com.gao.luna.semaphoretest E/TAG: Runnable1
04-03 15:33:07.306 17078-17078/com.gao.luna.semaphoretest E/TAG: Runnable0
04-03 15:33:07.306 17078-17785/com.gao.luna.semaphoretest E/TAG: num:0
04-03 15:33:07.308 17078-17786/com.gao.luna.semaphoretest E/TAG: num:0
04-03 15:33:07.308 17078-17787/com.gao.luna.semaphoretest E/TAG: num:0

这和我们希望见到的num从10打印到0时不相符的。

#1 Semaphore的使用

接着上面的代码，我们加入Semaphore来解决这个问题：

private final Semaphore semaphore = new Semaphore(0, true);

这里有个坑需要注意一下，比如说这个队列限制只有1个存在，那么在创建时要写0，写1的话，就会变成2个了···以此类推，要队列中有10个对象，需要写9···

在每次acquire后，semaphone将加入一个对象，在不release的情况下，这个对象是会一直存在的。

下面上代码：

    private final Semaphore semaphore = new Semaphore(0, true);

    private void Start() {
        for (int i = 10; i >= 0; i--) {
            try {
                Log.e("Semaphore", "semaphore.acquire();" + i);
                semaphore.acquire();
            } catch (Exception ex) {
                Log.e("Semaphore", ex.getMessage());
            }
            LogString = "num:" + i;
            printLogString(i);
        }
    }

    private void printLogString(final int delayTime) {
        new Thread(new Runnable() {
            @Override
            public void run() {
                Log.e("Semaphore", "semaphore.release();");
                Log.e("TAG", LogString);
                semaphore.release();
            }
        }).start();
    }

日志如下：

04-03 15:40:41.060 26200-26200/com.gao.luna.semaphoretest E/Semaphore: semaphore.release();
04-03 15:40:41.065 26200-26200/com.gao.luna.semaphoretest E/Semaphore: semaphore.acquire();10
04-03 15:40:41.078 26200-26200/com.gao.luna.semaphoretest E/Semaphore: semaphore.acquire();9
04-03 15:40:41.078 26200-26513/com.gao.luna.semaphoretest E/Semaphore: semaphore.release();
04-03 15:40:41.084 26200-26513/com.gao.luna.semaphoretest E/TAG: num:10
04-03 15:40:41.086 26200-26200/com.gao.luna.semaphoretest E/Semaphore: semaphore.acquire();8
04-03 15:40:41.086 26200-26514/com.gao.luna.semaphoretest E/Semaphore: semaphore.release();
04-03 15:40:41.086 26200-26514/com.gao.luna.semaphoretest E/TAG: num:9
04-03 15:40:41.086 26200-26200/com.gao.luna.semaphoretest E/Semaphore: semaphore.acquire();7
04-03 15:40:41.087 26200-26515/com.gao.luna.semaphoretest E/Semaphore: semaphore.release();
04-03 15:40:41.087 26200-26515/com.gao.luna.semaphoretest E/TAG: num:8
04-03 15:40:41.087 26200-26200/com.gao.luna.semaphoretest E/Semaphore: semaphore.acquire();6
04-03 15:40:41.088 26200-26516/com.gao.luna.semaphoretest E/Semaphore: semaphore.release();
04-03 15:40:41.088 26200-26516/com.gao.luna.semaphoretest E/TAG: num:7
04-03 15:40:41.088 26200-26200/com.gao.luna.semaphoretest E/Semaphore: semaphore.acquire();5
04-03 15:40:41.088 26200-26517/com.gao.luna.semaphoretest E/Semaphore: semaphore.release();
04-03 15:40:41.088 26200-26517/com.gao.luna.semaphoretest E/TAG: num:6
04-03 15:40:41.089 26200-26200/com.gao.luna.semaphoretest E/Semaphore: semaphore.acquire();4
04-03 15:40:41.089 26200-26518/com.gao.luna.semaphoretest E/Semaphore: semaphore.release();
04-03 15:40:41.089 26200-26518/com.gao.luna.semaphoretest E/TAG: num:5
04-03 15:40:41.090 26200-26200/com.gao.luna.semaphoretest E/Semaphore: semaphore.acquire();3
04-03 15:40:41.090 26200-26519/com.gao.luna.semaphoretest E/Semaphore: semaphore.release();
04-03 15:40:41.091 26200-26519/com.gao.luna.semaphoretest E/TAG: num:4
04-03 15:40:41.091 26200-26200/com.gao.luna.semaphoretest E/Semaphore: semaphore.acquire();2
04-03 15:40:41.092 26200-26520/com.gao.luna.semaphoretest E/Semaphore: semaphore.release();
04-03 15:40:41.092 26200-26520/com.gao.luna.semaphoretest E/TAG: num:3
04-03 15:40:41.092 26200-26200/com.gao.luna.semaphoretest E/Semaphore: semaphore.acquire();1
04-03 15:40:41.093 26200-26521/com.gao.luna.semaphoretest E/Semaphore: semaphore.release();
04-03 15:40:41.093 26200-26521/com.gao.luna.semaphoretest E/TAG: num:2
04-03 15:40:41.094 26200-26200/com.gao.luna.semaphoretest E/Semaphore: semaphore.acquire();0
04-03 15:40:41.094 26200-26522/com.gao.luna.semaphoretest E/Semaphore: semaphore.release();
04-03 15:40:41.094 26200-26522/com.gao.luna.semaphoretest E/TAG: num:1
04-03 15:40:41.095 26200-26523/com.gao.luna.semaphoretest E/Semaphore: semaphore.release();
04-03 15:40:41.095 26200-26523/com.gao.luna.semaphoretest E/TAG: num:0

这里我们可以很清晰的看到，num从10顺序变为了0。

#2 再加点延迟试试

现在，我们再在其中增加一些sleep代码，再来测试看看。

    private final Semaphore semaphore = new Semaphore(0, true);

    private void Start() {
        for (int i = 10; i >= 0; i--) {
            try {
                Log.e("Semaphore", "semaphore.acquire();" + i);
                semaphore.acquire();
            } catch (Exception ex) {
                Log.e("Semaphore", ex.getMessage());
            }
            LogString = "num:" + i;
            printLogString(i);
        }
    }

    private void printLogString(final int delayTime) {
        Log.e("TAG", "Runnable" + delayTime);
        new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(delayTime * 10);
                } catch (Exception ex) {
                    Log.e("delayTime", ex.getMessage());
                }
                Log.e("Semaphore", "semaphore.release();");
                Log.e("TAG", LogString);
                semaphore.release();
            }
        }).start();
    }

然后看看日志：

04-03 16:15:34.028 18148-18148/com.gao.luna.semaphoretest E/Semaphore: semaphore.release();
04-03 16:15:34.028 18148-18148/com.gao.luna.semaphoretest E/Semaphore: semaphore.acquire();10
04-03 16:15:34.028 18148-18148/com.gao.luna.semaphoretest E/TAG: Runnable10
04-03 16:15:34.029 18148-18148/com.gao.luna.semaphoretest E/Semaphore: semaphore.acquire();9
04-03 16:15:34.129 18148-18230/com.gao.luna.semaphoretest E/Semaphore: semaphore.release();
04-03 16:15:34.129 18148-18230/com.gao.luna.semaphoretest E/TAG: num:10
04-03 16:15:34.130 18148-18148/com.gao.luna.semaphoretest E/TAG: Runnable9
04-03 16:15:34.130 18148-18148/com.gao.luna.semaphoretest E/Semaphore: semaphore.acquire();8
04-03 16:15:34.221 18148-18232/com.gao.luna.semaphoretest E/Semaphore: semaphore.release();
04-03 16:15:34.221 18148-18232/com.gao.luna.semaphoretest E/TAG: num:9
04-03 16:15:34.221 18148-18148/com.gao.luna.semaphoretest E/TAG: Runnable8
04-03 16:15:34.222 18148-18148/com.gao.luna.semaphoretest E/Semaphore: semaphore.acquire();7
04-03 16:15:34.302 18148-18245/com.gao.luna.semaphoretest E/Semaphore: semaphore.release();
04-03 16:15:34.302 18148-18245/com.gao.luna.semaphoretest E/TAG: num:8
04-03 16:15:34.303 18148-18148/com.gao.luna.semaphoretest E/TAG: Runnable7
04-03 16:15:34.382 18148-18148/com.gao.luna.semaphoretest E/Semaphore: semaphore.acquire();6
04-03 16:15:34.453 18148-18251/com.gao.luna.semaphoretest E/Semaphore: semaphore.release();
04-03 16:15:34.453 18148-18251/com.gao.luna.semaphoretest E/TAG: num:7
04-03 16:15:34.453 18148-18148/com.gao.luna.semaphoretest E/TAG: Runnable6
04-03 16:15:34.462 18148-18148/com.gao.luna.semaphoretest E/Semaphore: semaphore.acquire();5
04-03 16:15:34.523 18148-18266/com.gao.luna.semaphoretest E/Semaphore: semaphore.release();
04-03 16:15:34.523 18148-18266/com.gao.luna.semaphoretest E/TAG: num:6
04-03 16:15:34.523 18148-18148/com.gao.luna.semaphoretest E/TAG: Runnable5
04-03 16:15:34.529 18148-18148/com.gao.luna.semaphoretest E/Semaphore: semaphore.acquire();4
04-03 16:15:34.579 18148-18267/com.gao.luna.semaphoretest E/Semaphore: semaphore.release();
04-03 16:15:34.579 18148-18267/com.gao.luna.semaphoretest E/TAG: num:5
04-03 16:15:34.579 18148-18148/com.gao.luna.semaphoretest E/TAG: Runnable4
04-03 16:15:34.635 18148-18148/com.gao.luna.semaphoretest E/Semaphore: semaphore.acquire();3
04-03 16:15:34.675 18148-18268/com.gao.luna.semaphoretest E/Semaphore: semaphore.release();
04-03 16:15:34.675 18148-18268/com.gao.luna.semaphoretest E/TAG: num:4
04-03 16:15:34.675 18148-18148/com.gao.luna.semaphoretest E/TAG: Runnable3
04-03 16:15:34.676 18148-18148/com.gao.luna.semaphoretest E/Semaphore: semaphore.acquire();2
04-03 16:15:34.706 18148-18272/com.gao.luna.semaphoretest E/Semaphore: semaphore.release();
04-03 16:15:34.706 18148-18272/com.gao.luna.semaphoretest E/TAG: num:3
04-03 16:15:34.706 18148-18148/com.gao.luna.semaphoretest E/TAG: Runnable2
04-03 16:15:34.733 18148-18148/com.gao.luna.semaphoretest E/Semaphore: semaphore.acquire();1
04-03 16:15:34.753 18148-18275/com.gao.luna.semaphoretest E/Semaphore: semaphore.release();
04-03 16:15:34.753 18148-18275/com.gao.luna.semaphoretest E/TAG: num:2
04-03 16:15:34.753 18148-18148/com.gao.luna.semaphoretest E/TAG: Runnable1
04-03 16:15:34.754 18148-18148/com.gao.luna.semaphoretest E/Semaphore: semaphore.acquire();0
04-03 16:15:34.764 18148-18280/com.gao.luna.semaphoretest E/Semaphore: semaphore.release();
04-03 16:15:34.764 18148-18280/com.gao.luna.semaphoretest E/TAG: num:1
04-03 16:15:34.764 18148-18148/com.gao.luna.semaphoretest E/TAG: Runnable0
04-03 16:15:34.765 18148-18281/com.gao.luna.semaphoretest E/Semaphore: semaphore.release();
04-03 16:15:34.765 18148-18281/com.gao.luna.semaphoretest E/TAG: num:0

仍然没有问题~~~

#3 需要注意的地方

如果使用了Handler，需要注意的是，Handler会返回主线程来进行调用，如果在Handler未执行时在acquire地方暂停了，那么Handler是不会执行的。需要特别留意。

下面是全部的代码：

package com.gao.luna.semaphoretest;

import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.util.Log;
import android.view.View;

import java.util.concurrent.Semaphore;


public class MainActivity extends AppCompatActivity {

    private String LogString;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(onClickListener);
    }

    View.OnClickListener onClickListener = new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Log.e("Semaphore", "semaphore.release();");
            semaphore.release();
            Start();
        }
    };

    private final Semaphore semaphore = new Semaphore(0, true);

    private void Start() {
        for (int i = 10; i >= 0; i--) {
            try {
                Log.e("Semaphore", "semaphore.acquire();" + i);
                semaphore.acquire();
            } catch (Exception ex) {
                Log.e("Semaphore", ex.getMessage());
            }
            LogString = "num:" + i;
            printLogString(i);
        }
    }

    private void printLogString(final int delayTime) {
        Log.e("TAG", "Runnable" + delayTime);
        new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(delayTime * 10);
                } catch (Exception ex) {
                    Log.e("delayTime", ex.getMessage());
                }
                Log.e("Semaphore", "semaphore.release();");
                Log.e("TAG", LogString);
                semaphore.release();
            }
        }).start();
    }
}