Android之Context的源码分析

1.Context是什么

从Context的字面意思是环境，上下文，在Android开发中，Context就是Android系统中每一个App的环境和上下文。对于刚入坑Android的小伙伴来说Context是一个神奇而又无所不能的东西。开发过程中会经常用到Context，比如最基础的启动一个Activity啦，发送一个广播啦，获取应用基本信息啦。今天我们就从源码里探究一下，Context到底还有哪些隐藏技能。
从源码层面理解，Context是一个抽象类，它里面定义了很多抽象方法，而Context的具体实现类是ContextWrapper和ContextImpl，通过简单的查看ContextWrapper和ContextImpl源码，发现这两货都是Context的代理类，Context中的抽象方法的真正实现在其子类里。
他们之间的继承关系如下：

Context.png
看完继承关系，我们来看下Context的源码。在源码中我标注了Context中重要的抽象方法的功能。

public abstract class Context {
    // 资源相关
    public abstract AssetManager getAssets();

    public abstract Resources getResources();

    // 包名管理
    public abstract PackageManager getPackageManager();

    // 获取内容提供者ContentResolver
    public abstract ContentResolver getContentResolver();

    // 获取app全局的Context实例
    public abstract Context getApplicationContext();

    // 反射相关
    public abstract ClassLoader getClassLoader();

    // 获取包名
    public abstract String getPackageName();

    // 获取应用信息
    public abstract ApplicationInfo getApplicationInfo();

    // 获取文件存储SharedPreference
    @Deprecated
    public File getSharedPrefsFile(String name) {
        return getSharedPreferencesPath(name);
    }

    public abstract SharedPreferences getSharedPreferences(String name, @PreferencesMode int mode);

    public abstract SharedPreferences getSharedPreferences(File file, @PreferencesMode int mode);

    // 移除文件存储SharedPreference
    public abstract boolean moveSharedPreferencesFrom(Context sourceContext, String name);

    // 删除文件存储SharedPreference
    public abstract boolean deleteSharedPreferences(String name);

    // 文件输入输出流
    public abstract FileInputStream openFileInput(String name)
            throws FileNotFoundException;

    public abstract FileOutputStream openFileOutput(String name, @FileMode int mode)
            throws FileNotFoundException;

    
    // 删除文件
    public abstract boolean deleteFile(String name);
    // 获取文件数据路径
    public abstract File getFileStreamPath(String name);
    // 获取SharedPreferences文件路径
    public abstract File getSharedPreferencesPath(String name);

    // 资源文件目录相关
    public abstract File getDataDir();

    public abstract File getFilesDir();

    public abstract File getNoBackupFilesDir();
    // 获取手机外置SD卡文件目录
    @Nullable
    public abstract File getExternalFilesDir(@Nullable String type);

    public abstract File[] getExternalFilesDirs(String type);

    public abstract File getDir(String name, @FileMode int mode);
    
    // SQLite数据库操作相关
    public abstract SQLiteDatabase openOrCreateDatabase(String name,

    public abstract SQLiteDatabase openOrCreateDatabase(String name,

    public abstract boolean moveDatabaseFrom(Context sourceContext, String name);

    public abstract boolean deleteDatabase(String name);

    public abstract File getDatabasePath(String name);

    public abstract String[] databaseList();

    // 与activity相关的函数
    public abstract void startActivity(@RequiresPermission Intent intent);

    @RequiresPermission(android.Manifest.permission.INTERACT_ACROSS_USERS_FULL)
    public void startActivityAsUser(@RequiresPermission Intent intent, UserHandle user) {
        throw new RuntimeException("Not implemented. Must override in a subclass.");
    }

    public abstract void startActivity(@RequiresPermission Intent intent,
                                       @Nullable Bundle options);

    @RequiresPermission(android.Manifest.permission.INTERACT_ACROSS_USERS_FULL)
    public void startActivityAsUser(@RequiresPermission Intent intent, @Nullable Bundle options,
                                    UserHandle userId) {
        throw new RuntimeException("Not implemented. Must override in a subclass.");
    }

    public void startActivityForResult(
            @NonNull String who, Intent intent, int requestCode, @Nullable Bundle options) {
        throw new RuntimeException("This method is only implemented for Activity-based Contexts. "
                + "Check canStartActivityForResult() before calling.");
    }

    public boolean canStartActivityForResult() {
        return false;
    }

    public abstract void startActivities(@RequiresPermission Intent[] intents);

    public abstract void startActivities(@RequiresPermission Intent[] intents, Bundle options);

    @RequiresPermission(android.Manifest.permission.INTERACT_ACROSS_USERS_FULL)
    public int startActivitiesAsUser(Intent[] intents, Bundle options, UserHandle userHandle) {
        throw new RuntimeException("Not implemented. Must override in a subclass.");
    }

    public abstract void startIntentSender(IntentSender intent, @Nullable Intent fillInIntent,
                                           @Intent.MutableFlags int flagsMask, @Intent.MutableFlags int flagsValues,
                                           int extraFlags) throws IntentSender.SendIntentException;

    public abstract void startIntentSender(IntentSender intent, @Nullable Intent fillInIntent,
                                           @Intent.MutableFlags int flagsMask, @Intent.MutableFlags int flagsValues,
                                           int extraFlags, @Nullable Bundle options) throws IntentSender.SendIntentException;
    //与Broadcast相关的函数
    public abstract void sendBroadcast(@RequiresPermission Intent intent);

    public abstract void sendBroadcast(@RequiresPermission Intent intent,
                                       @Nullable String receiverPermission);

    public abstract void sendBroadcastMultiplePermissions(Intent intent,
                                                          String[] receiverPermissions);
    ...

    @Deprecated
    @RequiresPermission(allOf = {
            android.Manifest.permission.INTERACT_ACROSS_USERS,
            android.Manifest.permission.BROADCAST_STICKY
    })
    public abstract void removeStickyBroadcastAsUser(@RequiresPermission Intent intent,
                                                     UserHandle user);

    @Nullable
    public abstract Intent registerReceiver(@Nullable BroadcastReceiver receiver,
                                            IntentFilter filter);


    ...

    @Nullable
    @RequiresPermission(android.Manifest.permission.INTERACT_ACROSS_USERS_FULL)
    public abstract Intent registerReceiverAsUser(BroadcastReceiver receiver,
                                                  UserHandle user, IntentFilter filter, @Nullable String broadcastPermission,
                                                  @Nullable Handler scheduler);


    public abstract void unregisterReceiver(BroadcastReceiver receiver);

    // 与Service相关
    @Nullable
    public abstract ComponentName startService(Intent service);

    // 启动前台Service
    @Nullable
    public abstract ComponentName startForegroundService(Intent service);

    @Nullable
    @RequiresPermission(android.Manifest.permission.INTERACT_ACROSS_USERS)
    public abstract ComponentName startForegroundServiceAsUser(Intent service, UserHandle user);

    public abstract boolean stopService(Intent service);

    @Nullable
    @RequiresPermission(android.Manifest.permission.INTERACT_ACROSS_USERS)
    public abstract ComponentName startServiceAsUser(Intent service, UserHandle user);

    @RequiresPermission(android.Manifest.permission.INTERACT_ACROSS_USERS)
    public abstract boolean stopServiceAsUser(Intent service, UserHandle user);

    public abstract boolean bindService(@RequiresPermission Intent service,
                                        @NonNull ServiceConnection conn, @BindServiceFlags int flags);

    @SystemApi
    @SuppressWarnings("unused")
    @RequiresPermission(android.Manifest.permission.INTERACT_ACROSS_USERS)
    public boolean bindServiceAsUser(@RequiresPermission Intent service, ServiceConnection conn,
                                     int flags, UserHandle user) {
        throw new RuntimeException("Not implemented. Must override in a subclass.");
    }

    @RequiresPermission(android.Manifest.permission.INTERACT_ACROSS_USERS)
    public boolean bindServiceAsUser(Intent service, ServiceConnection conn, int flags,
                                     Handler handler, UserHandle user) {
        throw new RuntimeException("Not implemented. Must override in a subclass.");
    }

    public abstract void unbindService(@NonNull ServiceConnection conn);

    public abstract boolean startInstrumentation(@NonNull ComponentName className,
                                                 @Nullable String profileFile, @Nullable Bundle arguments);

    // 系统相关
    public abstract @Nullable Object getSystemService(@ServiceName @NonNull String name);

    public final @Nullable <T> T getSystemService(@NonNull Class<T> serviceClass) {
        String serviceName = getSystemServiceName(serviceClass);
        return serviceName != null ? (T)getSystemService(serviceName) : null;
    }

    public abstract @Nullable String getSystemServiceName(@NonNull Class<?> serviceClass);

    // 检查权限
    @CheckResult(suggest="#enforcePermission(String,int,int,String)")
    @PackageManager.PermissionResult
    public abstract int checkPermission(@NonNull String permission, int pid, int uid);

    /** @hide */
    @PackageManager.PermissionResult
    public abstract int checkPermission(@NonNull String permission, int pid, int uid,
                                        IBinder callerToken);


    ...
    public abstract Context createPackageContext(String packageName,
                                                 @CreatePackageOptions int flags) throws PackageManager.NameNotFoundException;


    ...
}

2.Context有什么用

通过前面的源码我们可以看出Context的重要性，不但Android四大组件中的两个Activity和Service是其子类，而且四大组件的工作过程都与Context密切相关。Activity依赖Context的StartActivity系列函数去启动新的Activity，Service的整个生命周期函数都继承自Context，BroadcastReceiver的静态注册和注销以及发送各种类型的广播都依赖Context，内容提供者ContentProvider实例也需要通过Context去获取。除此之外，Context还提供了操作SQLite数据库，获取各类资源文件信息，获取应用相关的各种存储数据，以及获取应用的基本信息，还有检查各种权限的操作。总之，Context可以理解为Android应用的大总管，负责向Android应用提供各种服务。
附：Context的作用域
[图片上传中...(context的作用域.jpg-49fdc5-1561801607830-0)]
从上图我们可以发现Activity所持有的Context的作用域最广，无所不能。因为Activity继承自ContextThemeWrapper，而Application和Service继承自ContextWrapper，很显然ContextThemeWrapper在ContextWrapper的基础上又做了一些功能的扩展，使得其子类Activity更加强大。
参考文章：https://mp.weixin.qq.com/s/fmAPlgmqeHz0HRB6FTwRdg