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理解Android中不同的Context

news 2025/7/13 6:27:25

作者：两日的blog

Context是什么，有什么用

在Android开发中，Context是一个抽象类，它是Android应用程序环境的一部分。它提供了访问应用程序资源和执行各种操作的接口。可以说，Context是Android应用程序与系统环境进行交互的桥梁。

Context的作用包括：

访问应用程序资源：通过Context，可以获取应用程序的资源，如字符串、布局文件、图像等。这些资源可以在应用程序的各个组件中使用，例如Activity、Service、BroadcastReceiver等。
启动组件：通过Context，可以启动其他组件，如启动Activity、启动Service、发送广播等。它提供了访问系统服务的能力，如启动其他应用程序、发送系统广播等。
获取应用程序的上下文：通过Context，可以获取应用程序的上下文，如获取ApplicationContext，用于在整个应用程序中共享数据或执行全局操作。
访问系统服务：通过Context，可以访问各种系统服务，如获取系统级的服务（如传感器服务、位置服务）、访问设备功能（如摄像头、存储器）、执行网络操作等。
访问应用程序的文件：通过Context对象，可以获取应用程序的文件目录，创建、读取、写入和删除文件等操作。
处理资源生命周期：通过Context，可以管理应用程序资源的生命周期，如创建、销毁对象、注册和注销监听器等。它提供了一种机制，确保资源的正确使用和释放，避免内存泄漏等问题。

public abstract AssetManager getAssets();/*** Returns a Resources instance for the application's package.* <p>* <strong>Note:</strong> Implementations of this method should return* a Resources instance that is consistent with the AssetManager instance* returned by {@link #getAssets()}. For example, they should share the* same {@link Configuration} object.** @return a Resources instance for the application's package* @see #getAssets()*/
public abstract Resources getResources();/** Return PackageManager instance to find global package information. */
public abstract PackageManager getPackageManager();/** Return a ContentResolver instance for your application's package. */
public abstract ContentResolver getContentResolver();/*** Return the Looper for the main thread of the current process.  This is* the thread used to dispatch calls to application components (activities,* services, etc).* <p>* By definition, this method returns the same result as would be obtained* by calling {@link Looper#getMainLooper() Looper.getMainLooper()}.* </p>** @return The main looper.*/
public abstract Looper getMainLooper();/*** Return an {@link Executor} that will run enqueued tasks on the main* thread associated with this context. This is the thread used to dispatch* calls to application components (activities, services, etc).*/
public Executor getMainExecutor() {// This is pretty inefficient, which is why ContextImpl overrides itreturn new HandlerExecutor(new Handler(getMainLooper()));
}public abstract Context getApplicationContext();public final CharSequence getText(@StringRes int resId) {return getResources().getText(resId);
}/*** Returns a localized string from the application's package's* default string table.** @param resId Resource id for the string* @return The string data associated with the resource, stripped of styled*         text information.*/
@NonNull
public final String getString(@StringRes int resId) {return getResources().getString(resId);
}/*** Returns a localized formatted string from the application's package's* default string table, substituting the format arguments as defined in* {@link java.util.Formatter} and {@link java.lang.String#format}.** @param resId Resource id for the format string* @param formatArgs The format arguments that will be used for*                   substitution.* @return The string data associated with the resource, formatted and*         stripped of styled text information.*/
@NonNull
public final String getString(@StringRes int resId, Object... formatArgs) {return getResources().getString(resId, formatArgs);
}/*** Returns a color associated with a particular resource ID and styled for* the current theme.** @param id The desired resource identifier, as generated by the aapt*           tool. This integer encodes the package, type, and resource*           entry. The value 0 is an invalid identifier.* @return A single color value in the form 0xAARRGGBB.* @throws android.content.res.Resources.NotFoundException if the given ID*         does not exist.*/
@ColorInt
public final int getColor(@ColorRes int id) {return getResources().getColor(id, getTheme());
}/*** Returns a drawable object associated with a particular resource ID and* styled for the current theme.** @param id The desired resource identifier, as generated by the aapt*           tool. This integer encodes the package, type, and resource*           entry. The value 0 is an invalid identifier.* @return An object that can be used to draw this resource.* @throws android.content.res.Resources.NotFoundException if the given ID*         does not exist.*/
@Nullable
public final Drawable getDrawable(@DrawableRes int id) {return getResources().getDrawable(id, getTheme());
}/*** Returns a color state list associated with a particular resource ID and* styled for the current theme.** @param id The desired resource identifier, as generated by the aapt*           tool. This integer encodes the package, type, and resource*           entry. The value 0 is an invalid identifier.* @return A color state list.* @throws android.content.res.Resources.NotFoundException if the given ID*         does not exist.*/
@NonNull
public final ColorStateList getColorStateList(@ColorRes int id) {return getResources().getColorStateList(id, getTheme());
}/*** Set the base theme for this context.  Note that this should be called* before any views are instantiated in the Context (for example before* calling {@link android.app.Activity#setContentView} or* {@link android.view.LayoutInflater#inflate}).** @param resid The style resource describing the theme.*/
public abstract void setTheme(@StyleRes int resid);/** @hide Needed for some internal implementation...  not public because* you can't assume this actually means anything. */
@UnsupportedAppUsage
public int getThemeResId() {return 0;
}/*** Return the Theme object associated with this Context.*/
@ViewDebug.ExportedProperty(deepExport = true)
public abstract Resources.Theme getTheme();/*** Retrieve styled attribute information in this Context's theme.  See* {@link android.content.res.Resources.Theme#obtainStyledAttributes(int[])}* for more information.** @see android.content.res.Resources.Theme#obtainStyledAttributes(int[])*/
@NonNull
public final TypedArray obtainStyledAttributes(@NonNull @StyleableRes int[] attrs) {return getTheme().obtainStyledAttributes(attrs);
}/*** Retrieve styled attribute information in this Context's theme.  See* {@link android.content.res.Resources.Theme#obtainStyledAttributes(int, int[])}* for more information.** @see android.content.res.Resources.Theme#obtainStyledAttributes(int, int[])*/
@NonNull
public final TypedArray obtainStyledAttributes(@StyleRes int resid,@NonNull @StyleableRes int[] attrs) throws Resources.NotFoundException {return getTheme().obtainStyledAttributes(resid, attrs);
}/*** Retrieve styled attribute information in this Context's theme.  See* {@link android.content.res.Resources.Theme#obtainStyledAttributes(AttributeSet, int[], int, int)}* for more information.** @see android.content.res.Resources.Theme#obtainStyledAttributes(AttributeSet, int[], int, int)*/
@NonNull
public final TypedArray obtainStyledAttributes(@Nullable AttributeSet set, @NonNull @StyleableRes int[] attrs) {return getTheme().obtainStyledAttributes(set, attrs, 0, 0);
}/*** Retrieve styled attribute information in this Context's theme.  See* {@link android.content.res.Resources.Theme#obtainStyledAttributes(AttributeSet, int[], int, int)}* for more information.** @see android.content.res.Resources.Theme#obtainStyledAttributes(AttributeSet, int[], int, int)*/
@NonNull
public final TypedArray obtainStyledAttributes(@Nullable AttributeSet set,@NonNull @StyleableRes int[] attrs, @AttrRes int defStyleAttr,@StyleRes int defStyleRes) {return getTheme().obtainStyledAttributes(set, attrs, defStyleAttr, defStyleRes);
}

总之，Context在Android开发中具有重要的作用，它提供了访问应用程序资源、启动组件、访问系统服务以及处理资源生命周期的能力。开发者可以使用Context来实现各种应用程序功能和与系统环境的交互。

Context有哪些

Context 本身是一个抽象类，主要实现类为 ContextImpl，另外有子类 ContextWrapper 和 ContextThemeWrapper，另外还有其他由上述三个类引申出来的Context类，Application/Service/Activity，他们的继承关系如下：

`ContextImpl`/`ContextWrapper`/`ContextThemeWrapper`的区别

ContextImpl	ContextWrapper	ContextThemeWrapper
`ContextImpl`是`Context`的主要实现类，它提供了大部分`Context`的基本功能和行为。它是Android框架中真正的上下文实现类，用于处理应用程序的资源访问、组件启动、文件操作和系统服务等操作。	`ContextWrapper`是一个包装类，用于对现有的Context对象进行包装或修改其功能。它是`Context`的一个间接子类，可以通过继承`ContextWrapper`类来扩展`Context`的功能，例如添加自定义的行为或修改`Context`的行为。	`ContextThemeWrapper`：`ContextThemeWrapper`是`Context`的另一个包装类，它继承自`ContextWrapper`类。与`ContextWrapper`类似，`ContextThemeWrapper`也是用于包装现有的`Context`对象，但它还提供了自己的主题资源。通过`ContextThemeWrapper`，可以为特定的上下文设置不同的主题，以实现界面的样式和外观的变化。

ContextImpl

上文说到，Context本身是一个抽象类，主要的实现类就是ContextImpl，即Context的那些功能都是在ContexImpl中实现的，即ContextImpl实际承担着提供应用程序资源访问、组件启动和系统服务等功能的责任。

public class ContextImpl extends Context {private Resources mResources;private Theme mTheme;void setResources(Resources r) {if (r instanceof CompatResources) {((CompatResources) r).setContext(this);}mResources = r;}@Overridepublic Resources getResources() {return mResources;}@Overridepublic void setTheme(int resId) {synchronized (mSync) {if (mThemeResource != resId) {mThemeResource = resId;initializeTheme();}}}public Resources.Theme getTheme() {synchronized (mSync) {if (mTheme != null) {return mTheme;}mThemeResource = Resources.selectDefaultTheme(mThemeResource,getOuterContext().getApplicationInfo().targetSdkVersion);initializeTheme();return mTheme;}}private void initializeTheme() {if (mTheme == null) {mTheme = mResources.newTheme();}mTheme.applyStyle(mThemeResource, true);}// 其他方法的实现省略...
}

在ContextImpl，我们重点关注一下Resource及Theme的相关实现，ContextImpl中提供了getResources/setResources方法，用于获取Resources以及设置Resources，以提供资源的访问。

在getTheme/setTheme用于获取Theme以及设置Theme，以提供对主题的访问.

重点看一下getTheme()方法，该方法，会首先获取mThemeResource，这里直接选择的系统默认主题，系统会根据不同的sdk版本选择不同的默认主题。

@UnsupportedAppUsage
public static int selectDefaultTheme(int curTheme, int targetSdkVersion) {return selectSystemTheme(curTheme, targetSdkVersion,com.android.internal.R.style.Theme,com.android.internal.R.style.Theme_Holo,com.android.internal.R.style.Theme_DeviceDefault,com.android.internal.R.style.Theme_DeviceDefault_Light_DarkActionBar);
}/** @hide */
public static int selectSystemTheme(int curTheme, int targetSdkVersion, int orig, int holo,int dark, int deviceDefault) {if (curTheme != ID_NULL) {return curTheme;}if (targetSdkVersion < Build.VERSION_CODES.HONEYCOMB) {return orig;}if (targetSdkVersion < Build.VERSION_CODES.ICE_CREAM_SANDWICH) {return holo;}if (targetSdkVersion < Build.VERSION_CODES.N) {return dark;}return deviceDefault;
}

通过ContextImpl的实例，应用程序可以获取到Resources对象和Theme对象，从而实现对资源和主题的访问和处理。需要注意的是，这是一个简化的，实际的ContextImpl源码非常复杂，还涉及到处理上下文的生命周期、系统服务的获取等方面的逻辑。

ContextWrapper

ContextWrapper是一个包装类，内部包含一个mBase成员变量，所有的实现都是调用mBase的方法。

public class ContextWrapper extends Context {@UnsupportedAppUsageContext mBase;public ContextWrapper(Context base) {mBase = base;}@Overridepublic void setTheme(int resid) {mBase.setTheme(resid);}/** @hide */@Override@UnsupportedAppUsagepublic int getThemeResId() {return mBase.getThemeResId();}@Overridepublic Resources.Theme getTheme() {return mBase.getTheme();}@Overridepublic ClassLoader getClassLoader() {return mBase.getClassLoader();}@Overridepublic String getPackageName() {return mBase.getPackageName();}}

ContextThemeWrapper

ContextThemeWrapper继承自ContextWrapper，从名字中可以看出，该类主要是跟主题相关的包装类：

public class ContextThemeWrapper extends ContextWrapper {...@Overridepublic Resources getResources() {return getResourcesInternal();}private Resources getResourcesInternal() {if (mResources == null) {if (mOverrideConfiguration == null) {mResources = super.getResources();} else {final Context resContext = createConfigurationContext(mOverrideConfiguration);mResources = resContext.getResources();}}return mResources;}@Overridepublic void setTheme(int resid) {if (mThemeResource != resid) {mThemeResource = resid;initializeTheme();}}@Overridepublic Resources.Theme getTheme() {if (mTheme != null) {return mTheme;}mThemeResource = Resources.selectDefaultTheme(mThemeResource,getApplicationInfo().targetSdkVersion);initializeTheme();return mTheme;}@UnsupportedAppUsageprivate void initializeTheme() {final boolean first = mTheme == null;if (first) {mTheme = getResources().newTheme();final Resources.Theme theme = getBaseContext().getTheme();if (theme != null) {mTheme.setTo(theme);}}onApplyThemeResource(mTheme, mThemeResource, first);}...}

和ContextImpl相比较，ContextThemeWrapper中获取资源以及主题的代码有所不同，多了一个Configuration，其他行为大致一致。

另外在AppCompat中，默认的主题为Theme_AppCompat_Light，

package androidx.appcompat.view;public class ContextThemeWrapper extends ContextWrapper {... @Overridepublic Resources.Theme getTheme() {if (mTheme != null) {return mTheme;}if (mThemeResource == 0) {mThemeResource = R.style.Theme_AppCompat_Light;}initializeTheme();return mTheme;}
}

App中不同Context对象的Theme

我们在开发中，经常会用到各种Context，常用的有activity/application/applicationContext/baseContext，为了测试不同Context中Theme对象，我们编写如下代码：

class MainActivity : AppCompatActivity() {override fun onCreate(savedInstanceState: Bundle?) {super.onCreate(savedInstanceState)setContentView(R.layout.activity_main)printLog("baseContext is ${baseContext.themeResId} baseContext is $baseContext")printLog("application is ${application.themeResId} application is $application")printLog("applicationContext is ${applicationContext.themeResId} applicationContext is $applicationContext")printLog("activity is ${this.themeResId}")}private fun printLog(msg: String) {println("MainActivity themeResId in $msg")}
}

我们分别获取每个Context对应的themeResId，即每个Context中Theme对应的resId：

对代码运行结果我们有如下结论：

getApplication和getApplicationContext得到的是同一个Application实例对象；
Application对象中的themeResId 为0 ，Application其实也有主题的应用，毕竟主题样式都是针对UI元素的；
**Activity中的主题和getBaseContext**中的主题是不一样的，具体对应什么主题下文将进行探究
getBaseContext中得到的是ContextThemeWrapper，这点让我有点意外，之前的理解都是Activity启动时，会新建一个ContextImpl对象，在attachBaseContext中赋予Activity中的mBase，于是仔细研究一下发现，其实是AppCompatActivity做了替换：

//androidx.appcompat.app.AppCompatActivity
// AppCompatActivity重写了Activity中的attachBaseContext方法
@Override
protected void attachBaseContext(Context newBase) {super.attachBaseContext(getDelegate().attachBaseContext2(newBase));
}

我们看一下代理类AppCompatDelegateImpl中attachBaseContext2的实现：

//androidx.appcompat.app.AppCompatDelegateImpl@NonNull
@Override
@CallSuper
public Context attachBaseContext2(@NonNull final Context baseContext) {mBaseContextAttached = true;final int modeToApply = mapNightMode(baseContext, calculateNightMode());// If the base context is a ContextThemeWrapper (thus not an Application context)// and nobody's touched its Resources yet, we can shortcut and directly apply our// override configuration.if (sCanApplyOverrideConfiguration&& baseContext instanceof android.view.ContextThemeWrapper) {final Configuration config = createOverrideConfigurationForDayNight(baseContext, modeToApply, null);ContextThemeWrapperCompatApi17Impl.applyOverrideConfiguration((android.view.ContextThemeWrapper) baseContext, config);return baseContext;}// Again, but using the AppCompat version of ContextThemeWrapper.if (baseContext instanceof ContextThemeWrapper) {final Configuration config = createOverrideConfigurationForDayNight(baseContext, modeToApply, null);((ContextThemeWrapper) baseContext).applyOverrideConfiguration(config);return baseContext;}// We can't apply the configuration directly to the existing base context, so we need to// wrap it. We can't create a new configuration context since the app may rely on method// overrides or a specific theme -- neither of which are preserved when creating a// configuration context. Instead, we'll make a best-effort at wrapping the context and// rebasing the original theme.if (!sCanReturnDifferentContext) {return super.attachBaseContext2(baseContext);}Configuration configOverlay = null;final Configuration config = createOverrideConfigurationForDayNight(baseContext, modeToApply, configOverlay);//重点1：新建ContextThemeWrapper对象将传入的baseContext赋值给ContextWrapper中的mBase，// 并且ContextThemeWrapper中的主题为Theme_AppCompat_Empty// Next, we'll wrap the base context to ensure any method overrides or themes are left// intact. Since ThemeOverlay.AppCompat theme is empty, we'll get the base context's theme.final ContextThemeWrapper wrappedContext = new ContextThemeWrapper(baseContext,R.style.Theme_AppCompat_Empty);wrappedContext.applyOverrideConfiguration(config);// Check whether the base context has an explicit theme or is able to obtain one// from its outer context. If it throws an NPE because we're at an invalid point in app// initialization, we don't need to worry about rebasing under the new configuration.boolean needsThemeRebase;try {needsThemeRebase = baseContext.getTheme() != null;} catch (NullPointerException e) {needsThemeRebase = false;}if (needsThemeRebase) {// Attempt to rebase the old theme within the new configuration. This will only// work on SDK 23 and up, but it's unlikely that we're keeping the base theme// anyway so maybe nobody will notice. Note that calling getTheme() will clone// the base context's theme into the wrapped context's theme.ResourcesCompat.ThemeCompat.rebase(wrappedContext.getTheme());}return super.attachBaseContext2(wrappedContext);
}//androidx.appcompat.app.AppCompatDelegate
@NonNull
@CallSuper
public Context attachBaseContext2(@NonNull Context context) {
// 重点2，将上一步包装了baseContext的ContextThemeWrapper对象进一步赋值给Activity的mBaseattachBaseContext(context);return context;
}

最终AppCompatActivity中的mBase是包装了ContextImpl的ContextThemeWrapper对象，并且其主题为Theme_AppCompat_Empty

关于第三点，getBaseActivity和Activity中的主题到底是哪一个，我们可以根据resId和resources索引表resource.arsc（直接将apk文件拖到AndroidStudio中就可以看到该文件）找到：

2131755410和2131755474对应16进制为0x7f100192与0x7f1001d2

可以看到，getBaseActivity和Activity中的主题分别对应Theme_AppCompat_Empty 与我们在AndroidManifest.xml中设置的应用主题Theme.ThemeTest

总结

Context是Android应用程序与系统环境进行交互的桥梁，主要实现类是ContextImpl, 可以访问应用程序资源/启动组件/访问系统服务/访问应用程序的文件等，而Context可以分为三种：ContextImpl/ContextWrapper/ContextThemeWrapper，不同ContextImpl 是Context的主要实现类，ContextWrapper是简单的包装类，所有的实现都由其内部的mBase成员完成，ContextThemeWrapper继承自ContextWrapper ，它的主要继承者是Activity，和其他两个Context不同的是，他内部对应用资源和主题有不同的行为，在应用中使用跟主题相关的Context时，最好使用activity，而不要使用getBaseContext或者applictaion.

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