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一道面试题引发的对android中context的研究（三）-各种Context在ActivityThread中实例化过程源码分析

参考博客文章：http://blog.csdn.net/yanbober/article/details/45967639

接着上面的文章，继续分析，context的深入，这里进入源码的分析。

上面一篇文章的地址是：http://blog.csdn.net/u014315849/article/details/50392064

Context的实现是ContextImpl，Activity与Application和Service的创建都是在ActivityThread中完成的，至于在ActivityThread何时、怎样调运的关系后面会写文章分析，这里先直接给出结论，因为我们分析的重点是Context过程。

1 Activity中ContextImpl实例化源码分析

通过startActivity启动一个新的Activity时系统会回调ActivityThread的handleLaunchActivity()方法，该方法内部会调用performLaunchActivity()方法去创建一个Activity实例，然后回调Activity的onCreate()等方法。所以Activity的ContextImpl实例化是在ActivityThread类的performLaunchActivity方法中，如下：

<code class="language-java hljs  has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> Activity <span class="hljs-title" style="box-sizing: border-box;">performLaunchActivity</span>(ActivityClientRecord r, Intent customIntent) {
        ......
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//已经创建好新的activity实例</span>
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (activity != <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>) {
                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//创建一个Context对象</span>
                Context appContext = createBaseContextForActivity(r, activity);
                ......
                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//将上面创建的appContext传入到activity的attach方法</span>
                activity.attach(appContext, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>, getInstrumentation(), r.token,
                        r.ident, app, r.intent, r.activityInfo, title, r.parent,
                        r.embeddedID, r.lastNonConfigurationInstances, config,
                        r.referrer, r.voiceInteractor);
                ......
            }
        ......
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> activity;
    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li></ul>

看见上面performLaunchActivity的核心代码了吗？通过

createBaseContextForActivity(r, activity);

创建appContext，然后通过activity.attach设置值。

具体我们先看下createBaseContextForActivity方法源码，如下：

<code class="language-java hljs  has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> Context <span class="hljs-title" style="box-sizing: border-box;">createBaseContextForActivity</span>(ActivityClientRecord r,
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> Activity activity) {
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//实质就是new一个ContextImpl对象，调运ContextImpl的有参构造初始化一些参数    </span>
        ContextImpl appContext = ContextImpl.createActivityContext(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>, r.packageInfo, r.token);
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//特别特别留意这里！！！</span>
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//ContextImpl中有一个Context的成员叫mOuterContext，通过这条语句就可将当前新Activity对象赋值到创建的ContextImpl的成员mOuterContext（也就是让ContextImpl内部持有Activity）。</span>
        appContext.setOuterContext(activity);
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//创建返回值并且赋值</span>
        Context baseContext = appContext;
        ......
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//返回ContextImpl对象</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> baseContext;
    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li></ul>

再来看看activity.attach，也就是Activity中的attach方法，如下：

<code class="language-java hljs  has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> attach(Context context, ActivityThread aThread,
            Instrumentation instr, IBinder token, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> ident,
            Application application, Intent intent, ActivityInfo info,
            CharSequence title, Activity parent, String id,
            NonConfigurationInstances lastNonConfigurationInstances,
            Configuration config, String referrer, IVoiceInteractor voiceInteractor) {
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//特别特别留意这里！！！</span>
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//与上面createBaseContextForActivity方法中setOuterContext语句类似，不同的在于：</span>
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//通过ContextThemeWrapper类的attachBaseContext方法，将createBaseContextForActivity中实例化的ContextImpl对象传入到ContextWrapper类的mBase变量，这样ContextWrapper（Context子类）类的成员mBase就被实例化为Context的实现类ContextImpl</span>
        attachBaseContext(context);
        ......
    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li></ul>

通过上面Activity的Context实例化分析再结合上面Context继承关系可以看出：

Activity通过ContextWrapper的成员mBase来引用了一个ContextImpl对象，这样，Activity组件以后就可以通过这个ContextImpl对象来执行一些具体的操作（启动Service等）；同时ContextImpl类又通过自己的成员mOuterContext引用了与它关联的Activity，这样ContextImpl类也可以操作Activity。

SO，由此说明一个Activity就有一个Context，而且生命周期和Activity类相同（记住这句话，写应用就可以避免一些低级的内存泄漏问题）。

2 Service中ContextImpl实例化源码分析

写APP时我们通过startService或者bindService方法创建一个新Service时就会回调ActivityThread类的handleCreateService()方法完成相关数据操作（具体关于ActivityThread调运handleCreateService时机等细节分析与上面Activity雷同，后边文章会做分析）。具体handleCreateService方法代码如下：

<code class="language-java hljs  has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">handleCreateService</span>(CreateServiceData data) {
        ......
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//类似上面Activity的创建，这里创建service对象实例</span>
        Service service = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>;
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">try</span> {
            java.lang.ClassLoader cl = packageInfo.getClassLoader();
            service = (Service) cl.loadClass(data.info.name).newInstance();
        } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">catch</span> (Exception e) {
            ......
        }

        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">try</span> {
            ......
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//不做过多解释，创建一个Context对象</span>
            ContextImpl context = ContextImpl.createAppContext(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>, packageInfo);
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//特别特别留意这里！！！</span>
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//ContextImpl中有一个Context的成员叫mOuterContext，通过这条语句就可将当前新Service对象赋值到创建的ContextImpl的成员mOuterContext（也就是让ContextImpl内部持有Service）。</span>
            context.setOuterContext(service);

            Application app = packageInfo.makeApplication(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">false</span>, mInstrumentation);
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//将上面创建的context传入到service的attach方法</span>
            service.attach(context, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>, data.info.name, data.token, app,
                    ActivityManagerNative.getDefault());
            service.onCreate();
            ......
        } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">catch</span> (Exception e) {
            ......
        }
    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li></ul>

再来看看service.attach，也就是Service中的attach方法，如下：

<code class="language-java hljs  has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">attach</span>(
            Context context,
            ActivityThread thread, String className, IBinder token,
            Application application, Object activityManager) {
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//特别特别留意这里！！！</span>
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//与上面handleCreateService方法中setOuterContext语句类似，不同的在于：</span>
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//通过ContextWrapper类的attachBaseContext方法，将handleCreateService中实例化的ContextImpl对象传入到ContextWrapper类的mBase变量，这样ContextWrapper（Context子类）类的成员mBase就被实例化为Context的实现类ContextImpl</span>
        attachBaseContext(context);
        ......
    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li></ul>

可以看出步骤流程和Activity的类似，只是实现细节略有不同而已。

SO，由此说明一个Service就有一个Context，而且生命周期和Service类相同（记住这句话，写应用就可以避免一些低级的内存泄漏问题）。

3 Application中ContextImpl实例化源码分析

当我们写好一个APP以后每次重新启动时都会首先创建Application对象（每个APP都有一个唯一的全局Application对象，与整个APP的生命周期相同）。创建Application的过程也在ActivityThread类的handleBindApplication()方法完成相关数据操作（具体关于ActivityThread调运handleBindApplication时机等细节分析与上面Activity雷同，后边文章会做分析）。而ContextImpl的创建是在该方法中调运LoadedApk类的makeApplication方法中实现，LoadedApk类的makeApplication()方法中源代码如下：

<code class="language-java hljs  has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> Application <span class="hljs-title" style="box-sizing: border-box;">makeApplication</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">boolean</span> forceDefaultAppClass,
            Instrumentation instrumentation) {
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//只有新创建的APP才会走if代码块之后的剩余逻辑</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (mApplication != <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>) {
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> mApplication;
        }
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//即将创建的Application对象</span>
        Application app = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>;

        String appClass = mApplicationInfo.className;
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (forceDefaultAppClass || (appClass == <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>)) {
            appClass = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"android.app.Application"</span>;
        }

        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">try</span> {
            java.lang.ClassLoader cl = getClassLoader();
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (!mPackageName.equals(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"android"</span>)) {
                initializeJavaContextClassLoader();
            }
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//不做过多解释，创建一个Context对象</span>
            ContextImpl appContext = ContextImpl.createAppContext(mActivityThread, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>);
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//将Context传入Instrumentation类的newApplication方法</span>
            app = mActivityThread.mInstrumentation.newApplication(
                    cl, appClass, appContext);
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//特别特别留意这里！！！</span>
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//ContextImpl中有一个Context的成员叫mOuterContext，通过这条语句就可将当前新Application对象赋值到创建的ContextImpl的成员mOuterContext（也就是让ContextImpl内部持有Application）。</span>
            appContext.setOuterContext(app);
        } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">catch</span> (Exception e) {
            ......
        }
        ......
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> app;
    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li></ul>

接着看看Instrumentation.newApplication方法。如下源码：

<code class="language-java hljs  has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> Application <span class="hljs-title" style="box-sizing: border-box;">newApplication</span>(ClassLoader cl, String className, Context context)
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">throws</span> InstantiationException, IllegalAccessException, 
            ClassNotFoundException {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> newApplication(cl.loadClass(className), context);
    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

继续看重载两个参数的newApplication方法，如下：

<code class="language-java hljs  has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> Application <span class="hljs-title" style="box-sizing: border-box;">newApplication</span>(Class<?> clazz, Context context)
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">throws</span> InstantiationException, IllegalAccessException, 
            ClassNotFoundException {
        ......
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//继续传递context</span>
        app.attach(context);
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> app;
    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul>

继续看下Application类的attach方法，如下：

<code class="language-java hljs  has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> attach(Context context) {
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//特别特别留意这里！！！</span>
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//与上面makeApplication方法中setOuterContext语句类似，不同的在于：</span>
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//通过ContextWrapper类的attachBaseContext方法，将makeApplication中实例化的ContextImpl对象传入到ContextWrapper类的mBase变量，这样ContextWrapper（Context子类）类的成员mBase就被实例化为Application的实现类ContextImpl</span>
        attachBaseContext(context);
        ......
    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul>

可以看出步骤流程和Activity的类似，只是实现细节略有不同而已。

SO，由此说明一个Application就有一个Context，而且生命周期和Application类相同（然而一个App只有一个Application，而且与应用生命周期相同）。

4 应用程序APP各种Context访问资源的唯一性分析

你可能会有疑问，这么多Context都是不同实例，那么我们平时写App时通过context.getResources得到资源是不是就不是同一份呢？下面我们从源码来分析下，如下：

<code class="language-java hljs  has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">class ContextImpl extends Context {
    ......
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> ResourcesManager mResourcesManager;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> Resources mResources;
    ......
    <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> Resources <span class="hljs-title" style="box-sizing: border-box;">getResources</span>() {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> mResources;
    }
    ......
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li></ul>

看见没，有了上面分析我们可以很确定平时写的App中context.getResources方法获得的Resources对象就是上面ContextImpl的成员变量mResources。那我们追踪可以发现mResources的赋值操作如下：

<code class="language-java hljs  has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-title" style="box-sizing: border-box;">ContextImpl</span>(ContextImpl container, ActivityThread mainThread,
            LoadedApk packageInfo, IBinder activityToken, UserHandle user, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">boolean</span> restricted,
            Display display, Configuration overrideConfiguration) {
        ......
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//单例模式获取ResourcesManager对象</span>
        mResourcesManager = ResourcesManager.getInstance();
        ......
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//packageInfo对于一个APP来说只有一个，所以resources 是同一份</span>
        Resources resources = packageInfo.getResources(mainThread);
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (resources != <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>) {
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (activityToken != <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>
                    || displayId != Display.DEFAULT_DISPLAY
                    || overrideConfiguration != <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>
                    || (compatInfo != <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span> && compatInfo.applicationScale
                            != resources.getCompatibilityInfo().applicationScale)) {
                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//mResourcesManager是单例，所以resources是同一份</span>
                resources = mResourcesManager.getTopLevelResources(packageInfo.getResDir(),
                        packageInfo.getSplitResDirs(), packageInfo.getOverlayDirs(),
                        packageInfo.getApplicationInfo().sharedLibraryFiles, displayId,
                        overrideConfiguration, compatInfo, activityToken);
            }
        }
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//把resources赋值给mResources</span>
        mResources = resources;
        ......
    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li></ul>

由此可以看出在设备其他因素不变的情况下我们通过不同的Context实例得到的Resources是同一套资源。

PS一句，同样的分析方法也可以发现Context类的packageInfo对于一个应用来说也只有一份。感兴趣可以自行分析。

5 应用程序APP各种Context使用区分源码分析

5-1 先来解决getApplication和getApplicationContext的区别

很多人一直区分不开这两个方法的区别，这里从源码来分析一下，如下：

首先来看getApplication方法，你会发现Application与Context都没有提供该方法，这个方法是哪提供的呢？我们看下Activity与Service中的代码，可以发下如下：

<code class="language-java hljs  has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Activity</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">extends</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">ContextThemeWrapper</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">implements</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">LayoutInflater</span>.<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Factory2</span>,
        <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Window</span>.<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Callback</span>, <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">KeyEvent</span>.<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Callback</span>,
        <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">OnCreateContextMenuListener</span>, <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">ComponentCallbacks2</span>,
        <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Window</span>.<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">OnWindowDismissedCallback</span> {</span>
    ......
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> Application <span class="hljs-title" style="box-sizing: border-box;">getApplication</span>() {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> mApplication;
    }
    ......
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li></ul>      
<code class="language-java hljs  has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">abstract</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Service</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">extends</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">ContextWrapper</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">implements</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">ComponentCallbacks2</span> {</span>
    ......
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> Application <span class="hljs-title" style="box-sizing: border-box;">getApplication</span>() {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> mApplication;
    }
    ......
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul>

Activity和Service提供了getApplication，而且返回类型都是Application。这个mApplication都是在各自类的attach方法参数出入的，也就是说这个mApplication都是在ActivityThread中各自实例化时获取的makeApplication方法返回值。

所以不同的Activity和Service返回的Application均为同一个全局对象。

再来看看getApplicationContext方法，如下：

<code class="language-java hljs  has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">class ContextImpl extends Context {
    ......
    <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> Context <span class="hljs-title" style="box-sizing: border-box;">getApplicationContext</span>() {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> (mPackageInfo != <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>) ?
                mPackageInfo.getApplication() : mMainThread.getApplication();
    }
    ......
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li></ul>

可以看到getApplicationContext方法是Context的方法，而且返回值是Context类型，返回对象和上面通过Service或者Activity的getApplication返回的是一个对象。

所以说对于客户化的第三方应用来说两个方法返回值一样，只是返回值类型不同，还有就是依附的对象不同而已。

5-2 各种获取Context方法的差异及开发要点提示

可以看出来，Application的Context生命周期与应用程序完全相同。 

Activity或者Service的Context与他们各自类生命周期相同。

所以说对于Context使用不当会引起内存泄漏。

譬如一个单例模式的自定义数据库管理工具类需要传入一个Context，而这个数据库管理对象又需要在Activity中使用，如果我们传递Activity的Context就可能造成内存泄漏，所以需要传递Application的Context。

6 Context分析总结

到此整个Android应用的Context疑惑就完全解开了，同时也依据源码分析结果给出了平时开发APP中该注意的内存泄漏问题提示与解决方案。相信通过这一篇你在开发APP时对于Context的使用将不再迷惑。

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