What does @hide mean in the Android source code?

Question

For the Activity source code, line 3898 (close to the bottom):

/**
 * @hide
 */
public final boolean isResumed() {
    return mResumed;
}

What does @hide mean?

I found my public class ChildActivity extends Activity { ... } cannot use/see Activity.isResumed(). Is this normal? How can I access it?

Answer 1

Android has two types of APIs that are not accessible via SDK.

The first one is located in package com.android.internal. The second API type is a collection of classes and methods that are marked with the @hide Javadoc attribute.

Starting from Android 9 (API level 28), Google introduces new restrictions on the use of non-SDK interfaces, whether directly, via reflection, or via JNI. These restrictions are applied whenever an app references a non-SDK interface or attempts to obtain its handle using reflection or JNI.

But before API level 28, the hidden methods could still be accessed via Java reflection. The @hide attribute is just part of Javadoc (droiddoc also), so the @hide just simply means the method/class/field is excluded from the API docs.

For example, the checkUidPermission() method in ActivityManager.java uses @hide:

/** @hide */
public static int checkUidPermission(String permission, int uid) {
    try {
        return AppGlobals.getPackageManager()
                .checkUidPermission(permission, uid);
    } catch (RemoteException e) {
        // Should never happen, but if it does... deny!
        Slog.e(TAG, "PackageManager is dead?!?", e);
    }
    return PackageManager.PERMISSION_DENIED;
}

However, we can call it by reflection:

Class c;
c = Class.forName("android.app.ActivityManager");
Method m = c.getMethod("checkUidPermission", new Class[] {String.class, int.class});
Object o = m.invoke(null, new Object[]{"android.permission.READ_CONTACTS", 10010});