装饰模式(包装模式)—结构型-白红宇

装饰模式(包装模式)—结构型

阅读量：510 次

发布时间：2019-03-07

本文共 31656 字，大约阅读时间需要 105 分钟。

设计模式主要有23种，大致可分为三类：创建型，机构行，行为型；具体如下：

创建型

结构型

行为型

持续更新中...

结构型简介

结构型模式主要总结了一些类或对象组合在一起的经典结构，这些经典的结构可以解决特定应用场景的问题。结构型模式包括：代理模式、桥接模式、装饰器模式、适配器模式、门面模式、组合模式、享元模式。

装饰模式简介

装饰模式（Decorator Pattern）主要解决继承过于复杂的问题，通过组合来替代继承；也就是说是继承关系的一种替代方案之一；

UML图

UML图中各个部分代表的含义如下：

组件类

public abstract class Component {    public abstract void operate();}

组件具体实现类

public class ConcreteComponent extends Component {    @Override    public void operate() {            }}

抽象装饰类（如果装饰逻辑单一，只有一个的情况下我们可以省略该抽象类，直接是具体的装饰者）

public abstract class Decorator extends Component {    private Component mComponent;    public Decorator(Component mComponent) {        this.mComponent = mComponent;    }    @Override    public void operate() {        mComponent.operate();    }}

抽象装饰类具体实现类

public class ConcreteDecoratorA extends Decorator {    public ConcreteDecoratorA(Component mComponent) {        super(mComponent);    }    @Override    public void operate() {        operateA();        super.operate();        operateB();    }    public void operateA() {            }    public void operateB() {            }}

装饰模式在生活中无处不在，比如说牙膏包装盒，人各种穿着，不管怎么包装但是，人还是那个人本质没有变；

学过Java语言的熟悉IO流，BufferedInputStream，BufferedOutputStream,BufferedReader,BufferedWriter,DataInputStream，DataOutputStream;都是装饰者，都是对装饰设计模式的运用；感兴趣的可以自行查看源码；

JDK源码中的运用

下面以BufferedReader为例粘贴部分源码：

Writer：相当于UML图中的组件类

public abstract class Writer implements Appendable, Closeable, Flushable {       protected Object lock;    protected Writer() {        lock = this;    }        protected Writer(Object lock) {        if (lock == null) {            throw new NullPointerException("lock == null");        }        this.lock = lock;    }        public abstract void close() throws IOException;    public abstract void flush() throws IOException;        public void write(char[] buf) throws IOException {        write(buf, 0, buf.length);    }    public abstract void write(char[] buf, int offset, int count) throws IOException;    public void write(int oneChar) throws IOException {        synchronized (lock) {            char[] oneCharArray = new char[1];            oneCharArray[0] = (char) oneChar;            write(oneCharArray);        }    }    public void write(String str) throws IOException {        write(str, 0, str.length());    }        public void write(String str, int offset, int count) throws IOException {        if ((offset | count) < 0 || offset > str.length() - count) {            throw new StringIndexOutOfBoundsException(str, offset, count);        }        char[] buf = new char[count];        str.getChars(offset, offset + count, buf, 0);        synchronized (lock) {            write(buf, 0, buf.length);        }    }    public Writer append(char c) throws IOException {        write(c);        return this;    }        public Writer append(CharSequence csq) throws IOException {        if (csq == null) {            csq = "null";        }        write(csq.toString());        return this;    }    public Writer append(CharSequence csq, int start, int end) throws IOException {        if (csq == null) {            csq = "null";        }        write(csq.subSequence(start, end).toString());        return this;    }    boolean checkError() {        return false;    }}

FileWriter（OutputStreamWriter或者其他具体实现类）:相当于组件具体实现类（OutputStreamWriter 继承Writer）

public class FileWriter extends OutputStreamWriter {        public FileWriter(File file) throws IOException {        super(new FileOutputStream(file));    }    public FileWriter(File file, boolean append) throws IOException {        super(new FileOutputStream(file, append));    }        public FileWriter(FileDescriptor fd) {        super(new FileOutputStream(fd));    }       public FileWriter(String filename) throws IOException {        super(new FileOutputStream(new File(filename)));    }        public FileWriter(String filename, boolean append) throws IOException {        super(new FileOutputStream(filename, append));    }}

BufferedWriter:相当于UML图中的具体装饰类（BufferedWriter内部持有了Writer的引用）

public class BufferedWriter extends Writer {    private Writer out;    private char[] buf;    private int pos;    public BufferedWriter(Writer out) {        this(out, 8192);    }    public BufferedWriter(Writer out, int size) {        super(out);        if (size <= 0) {            throw new IllegalArgumentException("size <= 0");        }        this.out = out;        this.buf = new char[size];    }    @Override    public void close() throws IOException {        synchronized (lock) {            if (isClosed()) {                return;            }            Throwable thrown = null;            try {                flushInternal();            } catch (Throwable e) {                thrown = e;            }            buf = null;            try {                out.close();            } catch (Throwable e) {                if (thrown == null) {                    thrown = e;                }            }            out = null;            if (thrown != null) {                SneakyThrow.sneakyThrow(thrown);            }        }    }    @Override    public void flush() throws IOException {        synchronized (lock) {            checkNotClosed();            flushInternal();            out.flush();        }    }    private void checkNotClosed() throws IOException {        if (isClosed()) {            throw new IOException("BufferedWriter is closed");        }    }    private void flushInternal() throws IOException {        if (pos > 0) {            out.write(buf, 0, pos);        }        pos = 0;    }    private boolean isClosed() {        return out == null;    }    public void newLine() throws IOException {        write(System.lineSeparator());    }    @Override    public void write(char[] buffer, int offset, int count) throws IOException {        synchronized (lock) {            checkNotClosed();            if (buffer == null) {                throw new NullPointerException("buffer == null");            }            Arrays.checkOffsetAndCount(buffer.length, offset, count);            if (pos == 0 && count >= this.buf.length) {                out.write(buffer, offset, count);                return;            }            int available = this.buf.length - pos;            if (count < available) {                available = count;            }            if (available > 0) {                System.arraycopy(buffer, offset, this.buf, pos, available);                pos += available;            }            if (pos == this.buf.length) {                out.write(this.buf, 0, this.buf.length);                pos = 0;                if (count > available) {                    offset += available;                    available = count - available;                    if (available >= this.buf.length) {                        out.write(buffer, offset, available);                        return;                    }                    System.arraycopy(buffer, offset, this.buf, pos, available);                    pos += available;                }            }        }    }       @Override    public void write(int oneChar) throws IOException {        synchronized (lock) {            checkNotClosed();            if (pos >= buf.length) {                out.write(buf, 0, buf.length);                pos = 0;            }            buf[pos++] = (char) oneChar;        }    }    @Override    public void write(String str, int offset, int count) throws IOException {        synchronized (lock) {            checkNotClosed();            if (count <= 0) {                return;            }            if (offset < 0 || offset > str.length() - count) {                throw new StringIndexOutOfBoundsException(str, offset, count);            }            if (pos == 0 && count >= buf.length) {                char[] chars = new char[count];                str.getChars(offset, offset + count, chars, 0);                out.write(chars, 0, count);                return;            }            int available = buf.length - pos;            if (count < available) {                available = count;            }            if (available > 0) {                str.getChars(offset, offset + available, buf, pos);                pos += available;            }            if (pos == buf.length) {                out.write(this.buf, 0, this.buf.length);                pos = 0;                if (count > available) {                    offset += available;                    available = count - available;                    if (available >= buf.length) {                        char[] chars = new char[count];                        str.getChars(offset, offset + available, chars, 0);                        out.write(chars, 0, available);                        return;                    }                    str.getChars(offset, offset + available, buf, pos);                    pos += available;                }            }        }    }}

在Android源码也有对装饰模式的运用

Context类做Android的都不陌生，毫不夸张的说几乎每天都在用，四大组件每个里面都有他的直接或间接的引用到；Context本质上是一个抽象类，在装饰设计模式相当于抽象组件，在其内部定义了大量的抽象方法，比如我经常用到的startService和startActivity方法。通过查看Context源码是不是感觉很多方法很熟悉；

package android.content;//省略很多方法和其他包名等public abstract class Context {      public abstract AssetManager getAssets();    public abstract Resources getResources();    public abstract Looper getMainLooper();    @NonNull    public final CharSequence getText(@StringRes int resId) {        return getResources().getText(resId);    }    @NonNull    public final String getString(@StringRes int resId) {        return getResources().getString(resId);    }    @NonNull    public final String getString(@StringRes int resId, Object... formatArgs) {        return getResources().getString(resId, formatArgs);    }    @ColorInt    public final int getColor(@ColorRes int id) {        return getResources().getColor(id, getTheme());    }    @Nullable    public final Drawable getDrawable(@DrawableRes int id) {        return getResources().getDrawable(id, getTheme());    }    @NonNull    public final ColorStateList getColorStateList(@ColorRes int id) {        return getResources().getColorStateList(id, getTheme());    }    public abstract void setTheme(@StyleRes int resid);     @ViewDebug.ExportedProperty(deepExport = true)    public abstract Resources.Theme getTheme();    public abstract ClassLoader getClassLoader();    public abstract String getPackageName();        @UnsupportedAppUsage    public abstract String getBasePackageName();    @NonNull    public String getOpPackageName() {        throw new RuntimeException("Not implemented. Must override in a subclass.");    }    /** Return the full application info for this context's package. */    public abstract ApplicationInfo getApplicationInfo();    public abstract String getPackageResourcePath();    public abstract String getPackageCodePath();    @Deprecated    @UnsupportedAppUsage    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);    public abstract boolean moveSharedPreferencesFrom(Context sourceContext, String name);    public abstract boolean deleteSharedPreferences(String name);    /** @hide */    public abstract void reloadSharedPreferences();    public abstract FileInputStream openFileInput(String name)        throws FileNotFoundException;    public abstract FileOutputStream openFileOutput(String name, @FileMode int mode)        throws FileNotFoundException;    public abstract boolean deleteFile(String name);    public abstract File getFileStreamPath(String name);        public abstract File getSharedPreferencesPath(String name);    public abstract File getDataDir();    public abstract File getFilesDir();    public abstract File getNoBackupFilesDir();       @Nullable    public abstract File getExternalFilesDir(@Nullable String type);    public abstract File getObbDir();    public abstract File[] getObbDirs();    public abstract File getCacheDir();    public abstract File getCodeCacheDir();    @Nullable    public abstract File getExternalCacheDir();        @Nullable    @SystemApi    public abstract File getPreloadsFileCache();    public abstract File[] getExternalCacheDirs();    public abstract File[] getExternalMediaDirs();    public abstract String[] fileList();    public abstract File getDir(String name, @FileMode int mode);       public abstract SQLiteDatabase openOrCreateDatabase(String name,            @DatabaseMode int mode, CursorFactory factory);    public abstract SQLiteDatabase openOrCreateDatabase(String name,            @DatabaseMode int mode, CursorFactory factory,            @Nullable DatabaseErrorHandler errorHandler);    public abstract void startActivity(@RequiresPermission Intent intent);    public abstract void startActivity(@RequiresPermission Intent intent,            @Nullable Bundle options);    @UnsupportedAppUsage    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 abstract void startActivities(@RequiresPermission Intent[] intents);    public abstract void startActivities(@RequiresPermission Intent[] intents, Bundle options);    public abstract void sendBroadcast(@RequiresPermission Intent intent);    public abstract void sendBroadcast(@RequiresPermission Intent intent,            @Nullable String receiverPermission);    @SystemApi    public abstract void sendBroadcast(Intent intent,            @Nullable String receiverPermission,            @Nullable Bundle options);    @UnsupportedAppUsage    public abstract void sendBroadcast(Intent intent,            String receiverPermission, int appOp);    public abstract void sendOrderedBroadcast(@RequiresPermission Intent intent,            @Nullable String receiverPermission);    public abstract void sendOrderedBroadcast(@RequiresPermission @NonNull Intent intent,            @Nullable String receiverPermission, @Nullable BroadcastReceiver resultReceiver,            @Nullable Handler scheduler, int initialCode, @Nullable String initialData,            @Nullable Bundle initialExtras);    @SystemApi    public abstract void sendOrderedBroadcast(@NonNull Intent intent,            @Nullable String receiverPermission, @Nullable Bundle options,            @Nullable BroadcastReceiver resultReceiver, @Nullable Handler scheduler,            int initialCode, @Nullable String initialData, @Nullable Bundle initialExtras);    @Nullable    public abstract Intent registerReceiver(@Nullable BroadcastReceiver receiver,                                            IntentFilter filter);        @Nullable    public abstract Intent registerReceiver(@Nullable BroadcastReceiver receiver,                                            IntentFilter filter,                                            @RegisterReceiverFlags int flags);    @Nullable    public abstract Intent registerReceiver(BroadcastReceiver receiver,            IntentFilter filter, @Nullable String broadcastPermission,            @Nullable Handler scheduler);    @Nullable    public abstract Intent registerReceiver(BroadcastReceiver receiver,            IntentFilter filter, @Nullable String broadcastPermission,            @Nullable Handler scheduler, @RegisterReceiverFlags int flags);    @Nullable    @RequiresPermission(android.Manifest.permission.INTERACT_ACROSS_USERS_FULL)    @UnsupportedAppUsage    public abstract Intent registerReceiverAsUser(BroadcastReceiver receiver,            UserHandle user, IntentFilter filter, @Nullable String broadcastPermission,            @Nullable Handler scheduler);    public abstract void unregisterReceiver(BroadcastReceiver receiver);    @Nullable    public abstract ComponentName startService(Intent service);    @Nullable    public abstract ComponentName startForegroundService(Intent service);     public abstract boolean stopService(Intent service);    @Nullable    @RequiresPermission(android.Manifest.permission.INTERACT_ACROSS_USERS)    @UnsupportedAppUsage    public abstract ComponentName startServiceAsUser(Intent service, UserHandle user);    public abstract boolean bindService(@RequiresPermission Intent service,            @NonNull ServiceConnection conn, @BindServiceFlags int flags);    public boolean bindService(@RequiresPermission @NonNull Intent service,            @BindServiceFlags int flags, @NonNull @CallbackExecutor Executor executor,            @NonNull ServiceConnection conn) {        throw new RuntimeException("Not implemented. Must override in a subclass.");    }    @RequiresPermission(android.Manifest.permission.INTERACT_ACROSS_USERS)    @UnsupportedAppUsage    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);    /**     * Return the handle to a system-level service by name. The class of the     * returned object varies by the requested name. Currently available names     * are:     *     * 
   
         *  
    
      {@link #WINDOW_SERVICE} ("window")     *  
    
    
      The top-level window manager in which you can place custom     *  windows.  The returned object is a {@link android.view.WindowManager}.     *  
    
    
      {@link #LAYOUT_INFLATER_SERVICE} ("layout_inflater")     *  
    
    
      A {@link android.view.LayoutInflater} for inflating layout resources     *  in this context.     *  
    
    
      {@link #ACTIVITY_SERVICE} ("activity")     *  
    
    
      A {@link android.app.ActivityManager} for interacting with the     *  global activity state of the system.     *  
    
    
      {@link #POWER_SERVICE} ("power")     *  
    
    
      A {@link android.os.PowerManager} for controlling power     *  management.     *  
    
    
      {@link #ALARM_SERVICE} ("alarm")     *  
    
    
      A {@link android.app.AlarmManager} for receiving intents at the     *  time of your choosing.     *  
    
    
      {@link #NOTIFICATION_SERVICE} ("notification")     *  
    
    
      A {@link android.app.NotificationManager} for informing the user     *   of background events.     *  
    
    
      {@link #KEYGUARD_SERVICE} ("keyguard")     *  
    
    
      A {@link android.app.KeyguardManager} for controlling keyguard.     *  
    
    
      {@link #LOCATION_SERVICE} ("location")     *  
    
    
      A {@link android.location.LocationManager} for controlling location     *   (e.g., GPS) updates.     *  
    
    
      {@link #SEARCH_SERVICE} ("search")     *  
    
    
      A {@link android.app.SearchManager} for handling search.     *  
    
    
      {@link #VIBRATOR_SERVICE} ("vibrator")     *  
    
    
      A {@link android.os.Vibrator} for interacting with the vibrator     *  hardware.     *  
    
    
      {@link #CONNECTIVITY_SERVICE} ("connection")     *  
    
    
      A {@link android.net.ConnectivityManager ConnectivityManager} for     *  handling management of network connections.     *  
    
    
      {@link #IPSEC_SERVICE} ("ipsec")     *  
    
    
      A {@link android.net.IpSecManager IpSecManager} for managing IPSec on     *  sockets and networks.     *  
    
    
      {@link #WIFI_SERVICE} ("wifi")     *  
    
    
      A {@link android.net.wifi.WifiManager WifiManager} for management of Wi-Fi     *  connectivity.  On releases before NYC, it should only be obtained from an application     *  context, and not from any other derived context to avoid memory leaks within the calling     *  process.     *  
    
    
      {@link #WIFI_AWARE_SERVICE} ("wifiaware")     *  
    
    
      A {@link android.net.wifi.aware.WifiAwareManager WifiAwareManager} for management of     * Wi-Fi Aware discovery and connectivity.     *  
    
    
      {@link #WIFI_P2P_SERVICE} ("wifip2p")     *  
    
    
      A {@link android.net.wifi.p2p.WifiP2pManager WifiP2pManager} for management of     * Wi-Fi Direct connectivity.     * 
    
    
      {@link #INPUT_METHOD_SERVICE} ("input_method")     * 
    
    
      An {@link android.view.inputmethod.InputMethodManager InputMethodManager}     * for management of input methods.     * 
    
    
      {@link #UI_MODE_SERVICE} ("uimode")     * 
    
    
      An {@link android.app.UiModeManager} for controlling UI modes.     * 
    
    
      {@link #DOWNLOAD_SERVICE} ("download")     * 
    
    
      A {@link android.app.DownloadManager} for requesting HTTP downloads     * 
    
    
      {@link #BATTERY_SERVICE} ("batterymanager")     * 
    
    
      A {@link android.os.BatteryManager} for managing battery state     * 
    
    
      {@link #JOB_SCHEDULER_SERVICE} ("taskmanager")     * 
    
    
       A {@link android.app.job.JobScheduler} for managing scheduled tasks     * 
    
    
      {@link #NETWORK_STATS_SERVICE} ("netstats")     * 
    
    
      A {@link android.app.usage.NetworkStatsManager NetworkStatsManager} for querying network     * usage statistics.     * 
    
    
      {@link #HARDWARE_PROPERTIES_SERVICE} ("hardware_properties")     * 
    
    
      A {@link android.os.HardwarePropertiesManager} for accessing hardware properties.     * 
    
   
     *     * Note:  System services obtained via this API may be closely associated with     * the Context in which they are obtained from.  In general, do not share the     * service objects between various different contexts (Activities, Applications,     * Services, Providers, etc.)     *     * 
Note: Instant apps, for which {@link PackageManager#isInstantApp()} returns true,     * don't have access to the following system services: {@link #DEVICE_POLICY_SERVICE},     * {@link #FINGERPRINT_SERVICE}, {@link #KEYGUARD_SERVICE}, {@link #SHORTCUT_SERVICE},     * {@link #USB_SERVICE}, {@link #WALLPAPER_SERVICE}, {@link #WIFI_P2P_SERVICE},     * {@link #WIFI_SERVICE}, {@link #WIFI_AWARE_SERVICE}. For these services this method will     * return null.  Generally, if you are running as an instant app you should always     * check whether the result of this method is {@code null}.     *     * 
Note: When implementing this method, keep in mind that new services can be added on newer     * Android releases, so if you're looking for just the explicit names mentioned above, make sure     * to return {@code null} when you don't recognize the name — if you throw a     * {@link RuntimeException} exception instead, you're app might break on new Android releases.     *     * @param name The name of the desired service.     *     * @return The service or {@code null} if the name does not exist.     *     * @see #WINDOW_SERVICE     * @see android.view.WindowManager     * @see #LAYOUT_INFLATER_SERVICE     * @see android.view.LayoutInflater     * @see #ACTIVITY_SERVICE     * @see android.app.ActivityManager     * @see #POWER_SERVICE     * @see android.os.PowerManager     * @see #ALARM_SERVICE     * @see android.app.AlarmManager     * @see #NOTIFICATION_SERVICE     * @see android.app.NotificationManager     * @see #KEYGUARD_SERVICE     * @see android.app.KeyguardManager     * @see #LOCATION_SERVICE     * @see android.location.LocationManager     * @see #SEARCH_SERVICE     * @see android.app.SearchManager     * @see #SENSOR_SERVICE     * @see android.hardware.SensorManager     * @see #STORAGE_SERVICE     * @see android.os.storage.StorageManager     * @see #VIBRATOR_SERVICE     * @see android.os.Vibrator     * @see #CONNECTIVITY_SERVICE     * @see android.net.ConnectivityManager     * @see #WIFI_SERVICE     * @see android.net.wifi.WifiManager     * @see #AUDIO_SERVICE     * @see android.media.AudioManager     * @see #MEDIA_ROUTER_SERVICE     * @see android.media.MediaRouter     * @see #TELEPHONY_SERVICE     * @see android.telephony.TelephonyManager     * @see #TELEPHONY_SUBSCRIPTION_SERVICE     * @see android.telephony.SubscriptionManager     * @see #CARRIER_CONFIG_SERVICE     * @see android.telephony.CarrierConfigManager     * @see #INPUT_METHOD_SERVICE     * @see android.view.inputmethod.InputMethodManager     * @see #UI_MODE_SERVICE     * @see android.app.UiModeManager     * @see #DOWNLOAD_SERVICE     * @see android.app.DownloadManager     * @see #BATTERY_SERVICE     * @see android.os.BatteryManager     * @see #JOB_SCHEDULER_SERVICE     * @see android.app.job.JobScheduler     * @see #NETWORK_STATS_SERVICE     * @see android.app.usage.NetworkStatsManager     * @see android.os.HardwarePropertiesManager     * @see #HARDWARE_PROPERTIES_SERVICE     */    public abstract @Nullable Object getSystemService(@ServiceName @NonNull String name);    @SuppressWarnings("unchecked")    public final @Nullable 
    
      T getSystemService(@NonNull Class
     
       serviceClass) {        String serviceName = getSystemServiceName(serviceClass);        return serviceName != null ? (T)getSystemService(serviceName) : null;    }    @CheckResult(suggest="#enforceUriPermission(Uri,int,int,String)")    @PackageManager.PermissionResult    public abstract int checkUriPermission(Uri uri, int pid, int uid,            @Intent.AccessUriMode int modeFlags);    @PackageManager.PermissionResult    public abstract int checkUriPermission(Uri uri, int pid, int uid,            @Intent.AccessUriMode int modeFlags, IBinder callerToken);    public abstract Context createPackageContext(String packageName,            @CreatePackageOptions int flags) throws PackageManager.NameNotFoundException;    @UnsupportedAppUsage    public abstract Context createApplicationContext(ApplicationInfo application,            @CreatePackageOptions int flags) throws PackageManager.NameNotFoundException;       public abstract Context createContextForSplit(String splitName)            throws PackageManager.NameNotFoundException;    @TestApi    public UserHandle getUser() {        return android.os.Process.myUserHandle();    }    @TestApi    public @UserIdInt int getUserId() {        return android.os.UserHandle.myUserId();    }    public abstract Context createConfigurationContext(            @NonNull Configuration overrideConfiguration);    public abstract Context createDisplayContext(@NonNull Display display);    @SystemApi    public abstract Context createCredentialProtectedStorageContext();    public abstract DisplayAdjustments getDisplayAdjustments(int displayId);    @TestApi    public abstract Display getDisplay();    @TestApi    public abstract int getDisplayId();    public abstract void updateDisplay(int displayId);    public boolean isRestricted() {        return false;    }    public IApplicationThread getIApplicationThread() {        throw new RuntimeException("Not implemented. Must override in a subclass.");    }    public Handler getMainThreadHandler() {        throw new RuntimeException("Not implemented. Must override in a subclass.");    }}

其实现类是android.app.ContextImpl，ContextImpl继承Context，并实现了Context中的抽象方法；这里的ContextImpl相当于组件的具体实现类；

class ContextImpl extends Context {    @Override    public ComponentName startService(Intent service) {        warnIfCallingFromSystemProcess();        return startServiceCommon(service, false, mUser);    }    @Override    public ComponentName startForegroundService(Intent service) {        warnIfCallingFromSystemProcess();        return startServiceCommon(service, true, mUser);    }    @Override    public boolean stopService(Intent service) {        warnIfCallingFromSystemProcess();        return stopServiceCommon(service, mUser);    } @Override    public boolean bindService(Intent service, ServiceConnection conn, int flags) {        warnIfCallingFromSystemProcess();        return bindServiceCommon(service, conn, flags, null, mMainThread.getHandler(), null,                getUser());    }    @Override    public boolean bindService(            Intent service, int flags, Executor executor, ServiceConnection conn) {        warnIfCallingFromSystemProcess();        return bindServiceCommon(service, conn, flags, null, null, executor, getUser());    }     @Override    public void sendBroadcast(Intent intent) {        warnIfCallingFromSystemProcess();        String resolvedType = intent.resolveTypeIfNeeded(getContentResolver());        try {            intent.prepareToLeaveProcess(this);            ActivityManager.getService().broadcastIntent(                    mMainThread.getApplicationThread(), intent, resolvedType, null,                    Activity.RESULT_OK, null, null, null, AppOpsManager.OP_NONE, null, false, false,                    getUserId());        } catch (RemoteException e) {            throw e.rethrowFromSystemServer();        }    }@Override    public void sendBroadcast(Intent intent, String receiverPermission) {        warnIfCallingFromSystemProcess();        String resolvedType = intent.resolveTypeIfNeeded(getContentResolver());        String[] receiverPermissions = receiverPermission == null ? null                : new String[] {receiverPermission};        try {            intent.prepareToLeaveProcess(this);            ActivityManager.getService().broadcastIntent(                    mMainThread.getApplicationThread(), intent, resolvedType, null,                    Activity.RESULT_OK, null, null, receiverPermissions, AppOpsManager.OP_NONE,                    null, false, false, getUserId());        } catch (RemoteException e) {            throw e.rethrowFromSystemServer();        }    } @Override    public void sendOrderedBroadcast(Intent intent, String receiverPermission) {        warnIfCallingFromSystemProcess();        String resolvedType = intent.resolveTypeIfNeeded(getContentResolver());        String[] receiverPermissions = receiverPermission == null ? null                : new String[] {receiverPermission};        try {            intent.prepareToLeaveProcess(this);            ActivityManager.getService().broadcastIntent(                    mMainThread.getApplicationThread(), intent, resolvedType, null,                    Activity.RESULT_OK, null, null, receiverPermissions, AppOpsManager.OP_NONE,                    null, true, false, getUserId());        } catch (RemoteException e) {            throw e.rethrowFromSystemServer();        }    }}

Android中的Activity相比大家都很熟，从继承上说其实也是一个Content，但是Activity不是直接继承Content的，而是继承ContextThemeWrapper;

public class Activity extends ContextThemeWrapper {    @MainThread    @CallSuper    protected void onCreate(@Nullable Bundle savedInstanceState) {    }}

而ContextThemeWrapper又继承ContextWrapper

public class ContextThemeWrapper extends ContextWrapper {    @UnsupportedAppUsage    private int mThemeResource;    @UnsupportedAppUsage(maxTargetSdk = Build.VERSION_CODES.P, trackingBug = 123768723)    private Resources.Theme mTheme;    @UnsupportedAppUsage    private LayoutInflater mInflater;    private Configuration mOverrideConfiguration;    @UnsupportedAppUsage    private Resources mResources;}

最终ContextWrapper才继承Context,是不是感觉很复杂，其实这里就是一个典型的装饰模式的运用，ContextWrapper就是我们要找的装饰着，在ContextWrapper中有一个Context的引用。

public class ContextWrapper extends Context {    @UnsupportedAppUsage    Context mBase;    public ContextWrapper(Context base) {        mBase = base;    }    }

我们再看一下ContextWrapper内部的来自于组件的方法；

public class ContextWrapper extends Context {     @Override    public ComponentName startService(Intent service) {        return mBase.startService(service);    }    @Override    public boolean stopService(Intent name) {        return mBase.stopService(name);    }}

从上面的源码可以看出ContextWrapper中的startService方法也仅仅是调用了组件实现类（ContextImpl）中对应的方法而已，对于具体方法的包装扩展则有ContextWrapper的具体子类完成；比如我们的Service，Activity，Application.

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