Context类，应用程序环境的信息（即上下文，环境），是一个抽象类。

public abstract class Context {}

通读该类后发现几乎都是我们平时需要用context调用的抽象方法，比如getResources()，getPackageManager()，getContentResolver()，getApplicationContext()，obtainStyledAttributes()，getSharedPreferences(),startActivity(),checkPermission(),各种服务类型的声明的静态常量等等。还有调用Toast、Dialog需要传入。那么谁来实现这个功能的呢，既然上面Context是一个抽象类，那么肯定有他的实现类。

ContextImpl，Context所有抽象功能的实现类：

class ContextImpl extends Context {
    private Context mOuterContext;
    ......
}

Context的继承类ContextWrapper部分代码，代理Context的实现：

/**
 * Proxying implementation of Context that simply delegates all of its calls to
 * another Context.  Can be subclassed to modify behavior without changing
 * the original Context.
 */
public class ContextWrapper extends Context {
    Context mBase;

    public ContextWrapper(Context base) {
        mBase = base;
    }
    
    /**
     * Set the base context for this ContextWrapper.  All calls will then be
     * delegated to the base context.  Throws
     * IllegalStateException if a base context has already been set.
     * 
     * @param base The new base context for this wrapper.
     */
    protected void attachBaseContext(Context base) {
        if (mBase != null) {
            throw new IllegalStateException("Base context already set");
        }
        mBase = base;
    }
}

ContextThemeWrapper类代码：

/**
 * A context wrapper that allows you to modify or replace the theme of the
 * wrapped context.
 */
public class ContextThemeWrapper extends ContextWrapper {
    private int mThemeResource;
    private Resources.Theme mTheme;
    private LayoutInflater mInflater;
    private Configuration mOverrideConfiguration;
    private Resources mResources;

ContextThemeWrapper有setTheme()方法：

    @Override
    public void setTheme(int resid) {
        if (mThemeResource != resid) {
            mThemeResource = resid;
            initializeTheme();
        }
}

Activity类代码，因为Activity带窗口，需要设置主题，所以需继承于ContextThemeWrapper：

public class Activity extends ContextThemeWrapper
        implements LayoutInflater.Factory2,
        Window.Callback, KeyEvent.Callback,
        OnCreateContextMenuListener, ComponentCallbacks2,
        Window.OnWindowDismissedCallback, WindowControllerCallback,
        AutofillManager.AutofillClient {
    private static final String TAG = "Activity";
    private static final boolean DEBUG_LIFECYCLE = false;

    /** Standard activity result: operation canceled. */
    public static final int RESULT_CANCELED    = 0;
    /** Standard activity result: operation succeeded. */
    public static final int RESULT_OK           = -1;
    /** Start of user-defined activity results. */
public static final int RESULT_FIRST_USER   = 1;
}

Activity也复写该方法并最终给window设置主题

    @Override
    public void setTheme(int resid) {
        super.setTheme(resid);
        mWindow.setTheme(resid);
    }

Application类代码：

public class Application extends ContextWrapper implements ComponentCallbacks2 {
    private ArrayList<ComponentCallbacks> mComponentCallbacks =
            new ArrayList<ComponentCallbacks>();
    private ArrayList<ActivityLifecycleCallbacks> mActivityLifecycleCallbacks =
            new ArrayList<ActivityLifecycleCallbacks>();
    private ArrayList<OnProvideAssistDataListener> mAssistCallbacks = null;

    /** @hide */
    public LoadedApk mLoadedApk;

    public interface ActivityLifecycleCallbacks {
        void onActivityCreated(Activity activity, Bundle savedInstanceState);
        void onActivityStarted(Activity activity);
        void onActivityResumed(Activity activity);
        void onActivityPaused(Activity activity);
        void onActivityStopped(Activity activity);
        void onActivitySaveInstanceState(Activity activity, Bundle outState);
        void onActivityDestroyed(Activity activity);
  }
}

Service类代码：

public abstract class Service extends ContextWrapper implements ComponentCallbacks2 {
private static final String TAG = "Service";
…
}

继承关系图：

image.png

表述为：
ContextImpl和ContextWrapper继承自Context，ContextThemeWrapper、Application和Service继承自ContextWrapper，Activity继承自ContextThemeWrapper。ContextWrapper和ContextThemeWrapper都是Context的包装类，它们都含有Context类型的mBase对象，mBase具体指向的是ContextImpl，这样通过ContextWrapper和ContextThemeWrapper也可以使用Context的方法。ContextThemeWrapper中包含setTheme()和getTheme()等一些列主题相关的方法，所以需要设置主题的Activity需继承ContextThemeWrapper，让mWindow.setTheme()，而不需要主题的Service则继承ContextWrapper。

那么问题又来了

• 一个应用有多少个Context？

Application、Activity、Service都间接继承与Context，一个应用有且只有一个Application对象，那么一个应用Context总数为：

Context 数 = Application 数(1) + Activity 数(Activity以及所有子类) + Service 数(Service以及所有子类);

• 引用是该使用getApplicationcontext还是用Activity才不会引起内存泄漏？

当一个Context销毁的时候，却因为被引用导致销毁失败，导致内存泄露。如一个单例模式的Manager类需要传入一个Context，而这个Manager的对象又需要在Activity中使用，如果我们传递Activity的Context就可能造成内存泄漏，Application的Context对象是随着进程存在的，所以应该传递Application的Context。所以最终注意：尽量不要让生命周期长于Activity的对象引用Activity。

• getApplication()与getApplicationContext()的爱恨情仇

getApplication()方法属于Activity和Service，获取到的是Activity中的mApplication对象；getApplicationContext()方法属于ContextWrapper，获取到的是ContextWrapper中的Context类对象mBase。
在Activity和Service中可调用getApplication()获取context，而其他以外的组件获取context就只能调用getApplicationContext()。

来自Activity：

    /** Return the application that owns this activity. */
    public final Application getApplication() {
        return mApplication;
    }

来自ContextWrapper：

    @Override
    public Context getApplicationContext() {
        return mBase.getApplicationContext();
    }

打印它们的内存地址发现是相同的，所以getApplication()和getApplicationContext()获取到的是同一对象。

        Application mApplication = getApplication();
        Context mBase = getApplicationContext();

        Log.e("TAG","application " + mApplication);
        Log.e("TAG","context " + mBase);

image.png

关于Context的总结就到这里了，该篇文章会不断更新不断扩充详细，对Context有个更完整认识。