设计模式解析与实战之单列模式

买了大神关爱民与何红辉所著书籍《设计模式解析与实战》，观后有所感、有所悟。

单例模式之饿汉式：

public class Person {    /**     * 在课堂上不同身份的人说不通的话     */     public void say(){     }}

定义教导主任：

public class Director extends Person {    private static final Director mDirector=new Director();    public static Director getDirector(){        return mDirector;    }    @Override    public void say() {        super.say();        System.out.println("Director:"+"准备开始上课吧");    }}

定义上课老师：

public class Teacher extends Person {    @Override    public void say() {        super.say();        System.out.println("Teacher:"+"Stand up");    }}

定义上课学生：

public class Student extends Person {    @Override    public void say() {        super.say();        System.out.println("Student:"+"老师好");    }}

定义学校：

import java.util.ArrayList;public class School {    private ArrayList<Person> mPersonList=new ArrayList<>();    public void addPerson(Person mPerson){        mPersonList.add(mPerson);    }    public void showEveryOneSay(){        for(Person mPerson:mPersonList){            mPerson.say();        }    }}

下面是测试代码：

public class Test {    public static void main(String[] args) {        School mSchool =new School();        mSchool.addPerson(new Director());        mSchool.addPerson(new Teacher());        mSchool.addPerson(new Student());        mSchool.addPerson(new Student());        mSchool.showEveryOneSay();    }}

测试输出结果：

单例模式之懒汉式（synchronized 保证单例对象的唯一性）：

public class Person {    private static Person instance;    public Person() {    }    public static synchronized Person getInstance(){        if(instance==null){            instance=new Person();        }        return instance;    }}

DCL 实现单例，DCL的闪光点在getInstance方法上，两次判断为空，具体原因想知道更多详情请购买《设计模式解析与实战》自行了解吧，打字麻烦！！当然如果你耐不住性子可以通过以下博客内容找到答案：http://blog.csdn.net/liguangzhenghi/article/details/8076361（个人最喜欢用的方式）：

public class Person {    private static Person instance;    public Person() {    }    public static Person getInstance(){        if(instance==null){            synchronized (Person.class) {                if(instance==null){                    instance=new Person();                }            }        }        return instance;    }    }}

DCL优点：资源利用率高，缺点：第一次加载稍慢（基本可以忽略）。下面是静态内部类单例模式：

public class Person {    public Person() {    }    public static Person getInstance(){        return SingleHolder.instance;    }    private static class SingleHolder{        private static final Person instance=new Person();    }}

枚举单列（与其他单例模式不同，在反序列时不会重新new ）：

public enum PersonEnum {    student,teacher,director;    public void say(){    }}

通过容器实现单例（下面是对象为例的）：

import java.util.HashMap;import java.util.Map;public class PersonManager {    private static Map<String ,Object> map=new HashMap<String, Object>();    public PersonManager() {    }    public static <T> T putPersonService(T t){        map.put(t.getClass().getName(),t);        return t;    }    @SuppressWarnings("unchecked")    public static <T> T getPersonService(T t){        t=(T)map.get(t.getClass().getName());        return t;    }}

再补充一个特别的单例(Application,每个应用都有一个Application，应用启动就会创建，会调用onCreate方法，此时可以给instance赋值)：

import android.app.Application;public class MyApplication extends Application{    public static MyApplication instance;    @Override    public void onCreate() {        super.onCreate();        instance=this;    }}    <application        android:name="com.example.MyApplication"        android:allowBackup="true"        android:icon="@drawable/ic_launcher"        android:label="@string/app_name"        android:theme="@style/AppTheme" >

在android 源码中的运用例举如下：

  WindowManager manager=(WindowManager)getSystemService(Context.WINDOW_SERVICE);

而Context.WINDOW_SERVICE只是在常量类Context里面定义的键：

 /**     * Use with {@link #getSystemService} to retrieve a     * {@link android.view.WindowManager} for accessing the system's window     * manager.     *     * @see #getSystemService     * @see android.view.WindowManager     */    public static final String WINDOW_SERVICE = "window";

继续跟进Activity..

    @Override    public Object getSystemService(@ServiceName @NonNull String name) {        if (getBaseContext() == null) {            throw new IllegalStateException(                    "System services not available to Activities before onCreate()");        }        if (WINDOW_SERVICE.equals(name)) {            return mWindowManager;        } else if (SEARCH_SERVICE.equals(name)) {            ensureSearchManager();            return mSearchManager;        }        return super.getSystemService(name);    }

Activity继承ContextThemeWrapper,这里调用父类getSystemService，继续跟进..

@Override public Object getSystemService(String name) {    if (LAYOUT_INFLATER_SERVICE.equals(name)) {        if (mInflater == null) {            mInflater = LayoutInflater.from(getBaseContext()).cloneInContext(this);        }        return mInflater;    }    return getBaseContext().getSystemService(name);}

不断然而还没看到我想看的，只看到了一个抽象的Context对象，根据何红辉大神说的，我从Activity的ActivityThread入手（Activity的入口是ActivityThread的main函数）：

     public static void main(String[] args) {        ActivityThread thread = new ActivityThread();        thread.attach(false);        if (sMainThreadHandler == null) {            sMainThreadHandler = thread.getHandler();        }        if (false) {            Looper.myLooper().setMessageLogging(new                    LogPrinter(Log.DEBUG, "ActivityThread"));        }        // End of event ActivityThreadMain.        Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);        Looper.loop();        throw new RuntimeException("Main thread loop unexpectedly exited");    }

创建ActivityThread后调用attach函数传入参数fasle(非系统应用)，会通过Binder机制与ActivityManagerService通信，最终调用performLaunchActivity，然后调用createBaseContextForActivity方法:

   private Context createBaseContextForActivity(ActivityClientRecord r, final Activity activity) {        int displayId = Display.DEFAULT_DISPLAY;        try {            displayId = ActivityManagerNative.getDefault().getActivityDisplayId(r.token);        } catch (RemoteException e) {        }        ContextImpl appContext = ContextImpl.createActivityContext(                this, r.packageInfo, displayId, r.overrideConfig);        appContext.setOuterContext(activity);        Context baseContext = appContext;        .....................此处略...................     }

上面代码分析到Context实现类ContextImpl,Eclipse开发可能没法继续跟进了，android.jar里面也找不到，这时候需要https://github.com/android这里面去找源码了（framework层），换了Android Studio继续跟踪到了ContextImpl实现类，发现 ServiceManager.getService(key)方法的,没法跟进ServiceManager，只好github上https://github.com/android/platform_frameworks_base/blob/master/core/java/android/os/ServiceManager.java继续跟踪：

/* * Copyright (C) 2007 The Android Open Source Project * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * *      http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */package android.os;import com.android.internal.os.BinderInternal;import android.util.Log;import java.util.HashMap;import java.util.Map;/** @hide */public final class ServiceManager {    private static final String TAG = "ServiceManager";    private static IServiceManager sServiceManager;    private static HashMap<String, IBinder> sCache = new HashMap<String, IBinder>();    private static IServiceManager getIServiceManager() {        if (sServiceManager != null) {            return sServiceManager;        }        // Find the service manager        sServiceManager = ServiceManagerNative.asInterface(BinderInternal.getContextObject());        return sServiceManager;    }    /**     * Returns a reference to a service with the given name.     *      * @param name the name of the service to get     * @return a reference to the service, or <code>null</code> if the service doesn't exist     */    public static IBinder getService(String name) {        try {            IBinder service = sCache.get(name);            if (service != null) {                return service;            } else {                return getIServiceManager().getService(name);            }        } catch (RemoteException e) {            Log.e(TAG, "error in getService", e);        }        return null;    }    /**     * Place a new @a service called @a name into the service     * manager.     *      * @param name the name of the new service     * @param service the service object     */    public static void addService(String name, IBinder service) {        try {            getIServiceManager().addService(name, service, false);        } catch (RemoteException e) {            Log.e(TAG, "error in addService", e);        }    }    /**     * Place a new @a service called @a name into the service     * manager.     *      * @param name the name of the new service     * @param service the service object     * @param allowIsolated set to true to allow isolated sandboxed processes     * to access this service     */    public static void addService(String name, IBinder service, boolean allowIsolated) {        try {            getIServiceManager().addService(name, service, allowIsolated);        } catch (RemoteException e) {            Log.e(TAG, "error in addService", e);        }    }    /**     * Retrieve an existing service called @a name from the     * service manager.  Non-blocking.     */    public static IBinder checkService(String name) {        try {            IBinder service = sCache.get(name);            if (service != null) {                return service;            } else {                return getIServiceManager().checkService(name);            }        } catch (RemoteException e) {            Log.e(TAG, "error in checkService", e);            return null;        }    }    /**     * Return a list of all currently running services.     */    public static String[] listServices() throws RemoteException {        try {            return getIServiceManager().listServices();        } catch (RemoteException e) {            Log.e(TAG, "error in listServices", e);            return null;        }    }    /**     * This is only intended to be called when the process is first being brought     * up and bound by the activity manager. There is only one thread in the process     * at that time, so no locking is done.     *      * @param cache the cache of service references     * @hide     */    public static void initServiceCache(Map<String, IBinder> cache) {        if (sCache.size() != 0) {            throw new IllegalStateException("setServiceCache may only be called once");        }        sCache.putAll(cache);    }}

折腾这么久最后还是找到了想要的源码中单例模式的实战，继续拜读大神著作！！