Class.getResource(className)给我NullPointerException

Question

我已经建立了一个独立于平台的库,我想在J2SE和Android项目中使用它.在这个库中,我有一个Version类从清单加载其版本详细信息.在PC上运行良好,在Android上我得到了一个NullPointerException,我无法弄清楚为什么.

这是我的班级:

public class Version {

    private static int APPCODE = 12354;
    private static int MAJOR;
    private static int MINOR;
    private static char RELEASE;
    private static int BUILD;
    private static int PROTOCOL;

    static {
        try {
            Class clazz = Version.class;
            String className = clazz.getSimpleName() + ".class";
            String classPath = clazz.getResource(className).toString(); //NullPointerException
            if (classPath.startsWith("jar")) {
                String manifestPath = classPath.substring(0,
                        classPath.lastIndexOf("!") + 1) + "/META-INF/MANIFEST.MF";
                Manifest manifest = new Manifest(new URL(manifestPath).openStream());
                Attributes attr = manifest.getMainAttributes();
                //APPCODE = Integer.parseInt(attr.getValue("APPCODE"));
                MAJOR = Integer.parseInt(attr.getValue("MAJOR"));
                MINOR = Integer.parseInt(attr.getValue("MINOR"));
                RELEASE = attr.getValue("RELEASE").charAt(0);
                BUILD = Integer.parseInt(attr.getValue("BUILD"));
                PROTOCOL = Integer.parseInt(attr.getValue("PROTOCOL"));
            } else {
                System.err.println("Couldn't find manifest, reverting to test mode");
                MAJOR = 0;
                MINOR = 0;
                RELEASE = 'n';
                BUILD = 0;
                //APPCODE = 12354;
            }
        } catch (IOException e) {
            System.err.println("Failed to load manifest file. " + e);
            MAJOR = 0;
            MINOR = 0;
            RELEASE = '0';
            BUILD = 0;
            //APPCODE = 12354;
            PROTOCOL = 0;
        } catch (NumberFormatException e) {
            System.err.println("Failed to load manifest file. " + e);
            MAJOR = 0;
            MINOR = 0;
            RELEASE = '0';
            BUILD = 0;
            //APPCODE = 12354;
            PROTOCOL = 0;
        }
    }

    public static int getProtocol() {
        return PROTOCOL;
    }

    public static int getAppCode() {
        return APPCODE;
    }

    public static int getBuildNumber() {
        return BUILD;
    }

    public static int getMajor() {
        return MAJOR;
    }

    public static int getMinor() {
        return MINOR;
    }

    public static char getRelease() {
        return RELEASE;
    }
}

(请原谅这些System.err.println()线路,这些是用于PC上的调试).

为什么这在PC上运行良好但在Android上运行不正常？

完整堆栈跟踪:

03-12 22:13:11.687: E/AndroidRuntime(11780): FATAL EXCEPTION: main
03-12 22:13:11.687: E/AndroidRuntime(11780): java.lang.ExceptionInInitializerError
03-12 22:13:11.687: E/AndroidRuntime(11780):    at com.logandam.wififileshare.android.MainActivity.onCreate(MainActivity.java:24)
03-12 22:13:11.687: E/AndroidRuntime(11780):    at android.app.Activity.performCreate(Activity.java:4465)
03-12 22:13:11.687: E/AndroidRuntime(11780):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1052)
03-12 22:13:11.687: E/AndroidRuntime(11780):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1932)
03-12 22:13:11.687: E/AndroidRuntime(11780):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1993)
03-12 22:13:11.687: E/AndroidRuntime(11780):    at android.app.ActivityThread.access$600(ActivityThread.java:127)
03-12 22:13:11.687: E/AndroidRuntime(11780):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1159)
03-12 22:13:11.687: E/AndroidRuntime(11780):    at android.os.Handler.dispatchMessage(Handler.java:99)
03-12 22:13:11.687: E/AndroidRuntime(11780):    at android.os.Looper.loop(Looper.java:137)
03-12 22:13:11.687: E/AndroidRuntime(11780):    at android.app.ActivityThread.main(ActivityThread.java:4507)
03-12 22:13:11.687: E/AndroidRuntime(11780):    at java.lang.reflect.Method.invokeNative(Native Method)
03-12 22:13:11.687: E/AndroidRuntime(11780):    at java.lang.reflect.Method.invoke(Method.java:511)
03-12 22:13:11.687: E/AndroidRuntime(11780):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:790)
03-12 22:13:11.687: E/AndroidRuntime(11780):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:557)
03-12 22:13:11.687: E/AndroidRuntime(11780):    at dalvik.system.NativeStart.main(Native Method)
03-12 22:13:11.687: E/AndroidRuntime(11780): Caused by: java.lang.NullPointerException
03-12 22:13:11.687: E/AndroidRuntime(11780):    at com.logandam.wififileshare.net.Version.<clinit>(Version.java:30)
03-12 22:13:11.687: E/AndroidRuntime(11780):    ... 15 more

更新:className= Version.class在PC和Android上.在PC上使用getName()而不是getSimpleName()中断,但仍无法在Android上运行.

Answer 1

我不是 100% 确定，但我不认为这种机制在 Android 下的工作原理与在真正的 JVM 中的工作原理相同（因为 dalvik 本质上不是 JVM）。我认为（这里我可能是错的），在打包的 APK 中，JAR 不再存在，因此您的资源路径可能是错误的。JAR 的类将转换为 dex 文件格式并添加到该classes.dex文件中。因此，您无法解析资源，foo.bar.class因为它不存在于类路径中。

尝试将版本信息放入文本（或其他资源）文件中，并将其添加到 JAR 中并读取它。资源文件应该被正确添加，并且您应该能够使用该机制读取它。