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今天讲下四大组件添加android:permission属性，来实现限制访问。

你可以在<activity>中增加android:permission="string"，来限制调用方（caller）的访问，如果调用方未在AndroidManifest.xml中声明访问你的Activity的权限，则会报SecurityException。具体如下：

Enforcing Permissions in AndroidManifest.xml

You can apply high-level permissions restricting access to entire components of the system or application through your AndroidManifest.xml. To do this, include an android:permission attribute on the desired component, naming the permission that controls access
to it.
Activity permissions (applied to the <activity> tag) restrict who can start the associated activity. The permission is checked during Context.startActivity() and Activity.startActivityForResult(); if the caller does not have the required permission
then SecurityException is thrown from the call.
Service permissions (applied to the <service> tag) restrict who can start or bind to the associated service. The permission is checked during Context.startService(), Context.stopService() and Context.bindService(); if the caller does not have
the required permission then SecurityException is thrown from the call.
BroadcastReceiver permissions (applied to the <receiver> tag) restrict who can send broadcasts to the associated receiver. The permission is checked after Context.sendBroadcast() returns, as the system tries to deliver the submitted broadcast
to the given receiver. As a result, a permission failure will not result in an exception being thrown back to the caller; it will just not deliver the intent. In the same way, a permission can be supplied to Context.registerReceiver() to control who can broadcast
to a programmatically registered receiver. Going the other way, a permission can be supplied when calling Context.sendBroadcast() to restrict which BroadcastReceiver objects are allowed to receive the broadcast (see below).
ContentProvider permissions (applied to the <provider> tag) restrict who can access the data in a ContentProvider. (Content providers have an important additional security facility available to them called URI permissions which is described
later.) Unlike the other components, there are two separate permission attributes you can set: android:readPermission restricts who can read from the provider, and android:writePermission restricts who can write to it. Note that if a provider is protected
with both a read and write permission, holding only the write permission does not mean you can read from a provider. The permissions are checked when you first retrieve a provider (if you don't have either permission, a SecurityException will be thrown), and
as you perform operations on the provider. Using ContentResolver.query() requires holding the read permission; using ContentResolver.insert(), ContentResolver.update(), ContentResolver.delete() requires the write permission. In all of these cases, not holding
the required permission results in a SecurityException being thrown from the call.