Android应用程序上下文参数

Question

我有一个打包的库.jar，我想从中访问一个类。
但在param中这需要Context ，我是android dev中的新手。 试图在MainActivity上传递（ this ），但结果为null 。 你能解释为什么会发生这种情况以及如何解决？

这是示例代码：

Context paramContext = this;
    ApiHttpClient localApiHttpClient = (ApiHttpClient)paramContext.getSystemService("com.vine.api.ApiHttpClient");
    if (localApiHttpClient == null)
      localApiHttpClient = (ApiHttpClient)paramContext.getApplicationContext().getSystemService("com.vine.api.ApiHttpClient");
    if (localApiHttpClient == null)
        Log.d("loadState", "Result is null");

日志：

06-04 10:57:44.502: E/AndroidRuntime(6610): FATAL EXCEPTION: main
06-04 10:57:44.502: E/AndroidRuntime(6610): java.lang.ExceptionInInitializerError
06-04 10:57:44.502: E/AndroidRuntime(6610):     at java.lang.Class.classForName(Native Method)
06-04 10:57:44.502: E/AndroidRuntime(6610):     at java.lang.Class.forName(Class.java:217)
06-04 10:57:44.502: E/AndroidRuntime(6610):     at java.lang.Class.forName(Class.java:172)
06-04 10:57:44.502: E/AndroidRuntime(6610):     at com.example.dfgdf.MainActivity.onCreate(MainActivity.java:59)
06-04 10:57:44.502: E/AndroidRuntime(6610):     at android.app.Activity.performCreate(Activity.java:5008)
06-04 10:57:44.502: E/AndroidRuntime(6610):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)
06-04 10:57:44.502: E/AndroidRuntime(6610):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2140)
06-04 10:57:44.502: E/AndroidRuntime(6610):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2211)
06-04 10:57:44.502: E/AndroidRuntime(6610):     at android.app.ActivityThread.access$600(ActivityThread.java:143)
06-04 10:57:44.502: E/AndroidRuntime(6610):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1209)
06-04 10:57:44.502: E/AndroidRuntime(6610):     at android.os.Handler.dispatchMessage(Handler.java:99)
06-04 10:57:44.502: E/AndroidRuntime(6610):     at android.os.Looper.loop(Looper.java:137)
06-04 10:57:44.502: E/AndroidRuntime(6610):     at android.app.ActivityThread.main(ActivityThread.java:4965)
06-04 10:57:44.502: E/AndroidRuntime(6610):     at java.lang.reflect.Method.invokeNative(Native Method)
06-04 10:57:44.502: E/AndroidRuntime(6610):     at java.lang.reflect.Method.invoke(Method.java:511)
06-04 10:57:44.502: E/AndroidRuntime(6610):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:791)
06-04 10:57:44.502: E/AndroidRuntime(6610):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:558)
06-04 10:57:44.502: E/AndroidRuntime(6610):     at dalvik.system.NativeStart.main(Native Method)
06-04 10:57:44.502: E/AndroidRuntime(6610): Caused by: java.lang.NullPointerException
06-04 10:57:44.502: E/AndroidRuntime(6610):     at com.vine.android.vineApplication.getHttpUserAgent(Unknown Source)
06-04 10:57:44.502: E/AndroidRuntime(6610):     at com.vine.api.ApiHttpClient.(Unknown Source)

Answer 1

嗯，可能你需要将你的JAR添加到ClassLoader然后像Java一样创建Class.forName。

这里的例子

public static ClassLoader loadExternalJar(File file) throws Exception {
        if (file.exists()) {
            Method method = URLClassLoader.class.getDeclaredMethod("addURL", new Class[]{URL.class});
            method.setAccessible(true);
            ClassLoader retClassLoader;

            retClassLoader = Thread.currentThread().getContextClassLoader();

            method.invoke(retClassLoader, new Object[]{file.toURI().toURL()});
            return retClassLoader;
        } else {
            throw new Exception("File not found with JAR:"+file.getAbsolutePath());
        }
    }

然后尝试调用Class.forName（“com.vine.api.ApiHttpClient”）

尝试这个：

 try {
        Class<?> someApiClass = Class.forName("com.vine.api.ApiHttpClient");
        if (someApiClass != null) {
            ApiHttpClient localApiHttpClient = (ApiHttpClient) someApiClass
                    .newInstance();
            if (localApiHttpClient != null) {
                // TODO Oeprations
                Log.d("loadState", "Result is Good");
            } else {
                Log.d("loadState", "Result is null");
            }
        }
    } catch (ClassNotFoundException e) {
        e.printStackTrace();
    } catch (IllegalAccessException e) {
        e.printStackTrace();
    } catch (InstantiationException e) {
        e.printStackTrace();
    }