为什么Socket在运行时不是可闭合的实例？

Question

In an Android app, I added this code to onCreate()

    Closeable sss = new Socket();
    if (!(sss instanceof Closeable)) {
        throw new RuntimeException("Something unexpected happened");
    }

The imports are:

import java.io.Closeable;
import java.net.Socket;

The code compiles, but I am getting the exception:

E/AndroidRuntime( 8293): java.lang.RuntimeException: Unable to start activity...: java.lang.RuntimeException: Something unexpected happened
...
E/AndroidRuntime( 8293): Caused by: java.lang.RuntimeException: Something unexpected happened
...

In a different context, the glitch causes a java.lang.ArrayStoreException (namely, java.lang.ArrayStoreException: java.net.Socket cannot be stored in an array of type java.io.Closeable[] ).

Am I missing something? Any idea what to do?

EDIT Similarly, when DatagramSocket is used as MyClass<DatagramSocket> for MyClass<T extends Closeable> , it causes java.lang.IncompatibleClassChangeError: interface not implemented .

Answer 1

This issue occurs on Android API levels prior to 19. On affected versions, Socket does not implement Closeable.

Sources:

• Android: Exception using Closeable interface with Socket
• https://code.google.com/p/android/issues/detail?id=62909

Answer 2

Socket is not itself closeable, but both of its streams are. And closing either stream will close the Socket

Answer 3

For future readers, I want to clarify something that was assumed to be evident by everyone in this discussion:

How could I assign Closeable sss = new Socket(); if Socket is not Closeable ?

The API requirements specified in AndroidManifest.xml are:

<uses-sdk
    android:minSdkVersion="14"
    android:targetSdkVersion="19" />

and the device reports

android.os.Build.VERSION.SDK_INT=16

The code was compiled for API level 19, this is why it was successfully compiled. But it was run on API level 16, and this is why it failed.

In other words, the compile-time check was performed for API19 on one computer (the desktop PC) while the run-time check was performed for API16 on another computer (the handheld device).