Android中利用反射机制创建实例的代码

最新推荐文章于 2019-02-19 15:34:00 发布

原创最新推荐文章于 2019-02-19 15:34:00 发布 · 676 阅读

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#源码 #多态

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反射机制在Android编程中提供了代码的灵活性，特别是在利用多态性启动不同服务时。例如，SystemServiceManager的startService方法通过反射来动态开启如ShortcutService.Lifecycle、LauncherAppsService和MediaProjectionManagerService等服务。

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反射机制创建对象的最大优点是利用了Java语言的多态性，这样使得代码更加灵活。在观看源码的时候有这样一段代码就是典型的例子，可以借鉴：

出自SystemServiceManager的startService方法，这样在SystemServer中可以动态的开启想要开启的服务。

... ...

mSystemServiceManager.startService(ShortcutService.Lifecycle.class);
mSystemServiceManager.startService(LauncherAppsService.class);
mSystemServiceManager.startService(MediaProjectionManagerService.class);

... ...

    /**
     * Creates and starts a system service. The class must be a subclass of
     * {@link com.android.server.SystemService}.
     *
     * @param serviceClass A Java class that implements the SystemService interface.
     * @return The service instance, never null.
     * @throws RuntimeException if the service fails to start.
     */
    @SuppressWarnings("unchecked")
    public <T extends SystemService> T startService(Class<T> serviceClass) {
        try {
            final String name = serviceClass.getName();
            Slog.i(TAG, "Starting " + name);
            Trace.traceBegin(Trace.TRACE_TAG_SYSTEM_SERVER, "StartService " + name);

            // Create the service.
            if (!SystemService.class.isAssignableFrom(serviceClass)) {
                throw new RuntimeException("Failed to create " + name
                        + ": service must extend " + SystemService.class.getName());
            }
            final T service;
            try {
                Constructor<T> constructor = serviceClass.getConstructor(Context.class);
                service = constructor.newInstance(mContext);
            } catch (InstantiationException ex) {
                throw new RuntimeException("Failed to create service " + name
                        + ": service could not be instantiated", ex);
            } catch (IllegalAccessException ex) {
                throw new RuntimeException("Failed to create service " + name
                        + ": service must have a public constructor with a Context argument", ex);
            } catch (NoSuchMethodException ex) {
                throw new RuntimeException("Failed to create service " + name
                        + ": service must have a public constructor with a Context argument", ex);
            } catch (InvocationTargetException ex) {
                throw new RuntimeException("Failed to create service " + name
                        + ": service constructor threw an exception", ex);
            }

            // Register it.
            mServices.add(service);

            // Start it.
            try {
                service.onStart();
            } catch (RuntimeException ex) {
                throw new RuntimeException("Failed to start service " + name
                        + ": onStart threw an exception", ex);
            }
            return service;
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
        }
    }