java - 在 Android 应用程序中使用属性文件时出现 NullPointerException

Question

我想使用属性文件来读取我的 Android 应用程序中的一些配置数据。当我将属性文件放置在处理程序的类文件夹中时，一切正常，但我想将属性文件放置在项目根目录中。我已经尝试使用相对路径(../../../ 等)，但它不起作用。

有没有一种“简单”的方法可以做到这一点(/config.properties也不起作用)？

这是我的代码:

    private Properties prop;

    public PropertiesHandler() {
        this.prop = new Properties();
        InputStream input = null;

        try {
            input = getClass().getResourceAsStream("config.properties");
            //input = new FileInputStream("config.properties");
            prop.load(input);
        } catch (IOException ex) {
            ex.printStackTrace();
        } finally {
            if (input != null) {
                try {
                    input.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }

任何帮助将不胜感激。

编辑:

不幸的是，我在应用建议的解决方案时遇到了此异常:

10-08 15:10:05.903: E/AndroidRuntime(967): FATAL EXCEPTION: main
10-08 15:10:05.903: E/AndroidRuntime(967): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.dev.app1234/com.dev.app1234.Register}: java.lang.NullPointerException
10-08 15:10:05.903: E/AndroidRuntime(967):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1983)
10-08 15:10:05.903: E/AndroidRuntime(967):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
10-08 15:10:05.903: E/AndroidRuntime(967):  at android.app.ActivityThread.access$600(ActivityThread.java:130)
10-08 15:10:05.903: E/AndroidRuntime(967):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
10-08 15:10:05.903: E/AndroidRuntime(967):  at android.os.Handler.dispatchMessage(Handler.java:99)
10-08 15:10:05.903: E/AndroidRuntime(967):  at android.os.Looper.loop(Looper.java:137)
10-08 15:10:05.903: E/AndroidRuntime(967):  at android.app.ActivityThread.main(ActivityThread.java:4745)
10-08 15:10:05.903: E/AndroidRuntime(967):  at java.lang.reflect.Method.invokeNative(Native Method)
10-08 15:10:05.903: E/AndroidRuntime(967):  at java.lang.reflect.Method.invoke(Method.java:511)
10-08 15:10:05.903: E/AndroidRuntime(967):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
10-08 15:10:05.903: E/AndroidRuntime(967):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
10-08 15:10:05.903: E/AndroidRuntime(967):  at dalvik.system.NativeStart.main(Native Method)
10-08 15:10:05.903: E/AndroidRuntime(967): Caused by: java.lang.NullPointerException
10-08 15:10:05.903: E/AndroidRuntime(967):  at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:101)
10-08 15:10:05.903: E/AndroidRuntime(967):  at com.dev.app1234.Register.<init>(Register.java:68)
10-08 15:10:05.903: E/AndroidRuntime(967):  at java.lang.Class.newInstanceImpl(Native Method)
10-08 15:10:05.903: E/AndroidRuntime(967):  at java.lang.Class.newInstance(Class.java:1319)
10-08 15:10:05.903: E/AndroidRuntime(967):  at android.app.Instrumentation.newActivity(Instrumentation.java:1053)
10-08 15:10:05.903: E/AndroidRuntime(967):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1974)
10-08 15:10:05.903: E/AndroidRuntime(967):  ... 11 more

这是我的代码:

public PropertiesHandler(Context context) {
        this.prop = new Properties();
        InputStream input = null;

        try {
            input = context.getAssets().open("config.properties");

我这样调用它:

private final String URL = new PropertiesHandler(this.getApplicationContext()).getUrl()
                + "url.php";

编辑2:

代码:

private final String URL = new PropertiesHandler(Register.this).getUrl()
            + "url.php";

堆栈跟踪:

10-08 20:45:14.421: E/AndroidRuntime(1024): FATAL EXCEPTION: main
10-08 20:45:14.421: E/AndroidRuntime(1024): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.dev.app1234/com.dev.app1234.Register}: java.lang.NullPointerException
10-08 20:45:14.421: E/AndroidRuntime(1024):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1983)
10-08 20:45:14.421: E/AndroidRuntime(1024):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
10-08 20:45:14.421: E/AndroidRuntime(1024):     at android.app.ActivityThread.access$600(ActivityThread.java:130)
10-08 20:45:14.421: E/AndroidRuntime(1024):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
10-08 20:45:14.421: E/AndroidRuntime(1024):     at android.os.Handler.dispatchMessage(Handler.java:99)
10-08 20:45:14.421: E/AndroidRuntime(1024):     at android.os.Looper.loop(Looper.java:137)
10-08 20:45:14.421: E/AndroidRuntime(1024):     at android.app.ActivityThread.main(ActivityThread.java:4745)
10-08 20:45:14.421: E/AndroidRuntime(1024):     at java.lang.reflect.Method.invokeNative(Native Method)
10-08 20:45:14.421: E/AndroidRuntime(1024):     at java.lang.reflect.Method.invoke(Method.java:511)
10-08 20:45:14.421: E/AndroidRuntime(1024):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
10-08 20:45:14.421: E/AndroidRuntime(1024):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
10-08 20:45:14.421: E/AndroidRuntime(1024):     at dalvik.system.NativeStart.main(Native Method)
10-08 20:45:14.421: E/AndroidRuntime(1024): Caused by: java.lang.NullPointerException
10-08 20:45:14.421: E/AndroidRuntime(1024):     at android.content.ContextWrapper.getAssets(ContextWrapper.java:75)
10-08 20:45:14.421: E/AndroidRuntime(1024):     at com.dev.app1234.model.PropertiesHandler.<init>(PropertiesHandler.java:25)
10-08 20:45:14.421: E/AndroidRuntime(1024):     at com.dev.app1234.Register.<init>(Register.java:68)
10-08 20:45:14.421: E/AndroidRuntime(1024):     at java.lang.Class.newInstanceImpl(Native Method)
10-08 20:45:14.421: E/AndroidRuntime(1024):     at java.lang.Class.newInstance(Class.java:1319)
10-08 20:45:14.421: E/AndroidRuntime(1024):     at android.app.Instrumentation.newActivity(Instrumentation.java:1053)
10-08 20:45:14.421: E/AndroidRuntime(1024):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1974)
10-08 20:45:14.421: E/AndroidRuntime(1024):     ... 11 more

Answer 1

此调用:

private final String URL = new PropertiesHandler(Register.this).getUrl() + "url.php";

需要放置在 Activity 的 onCreate() 内方法。就像这样:

class Register extends Activity {

    private String URL;

    // ...

    protected void onCreate(Bundle savedInstanceState) {
        // ...
        URL = new PropertiesHandler(Register.this).getUrl() + "url.php";
        // ...
    }
}

通过在创建 Activity 之前创建自定义类，您可以有效地传递 null在此代码中:new PropertiesHandler(Register.this) ，因此堆栈跟踪中会出现空指针错误。

像这样的错误经常发生，因为 Android 中许多有用的系统调用都需要 Context 。发生这种情况的一个很好的线索是在堆栈跟踪中看到对 <init> 的引用。虚方法，指类中未包含在其他方法中的所有变量初始值设定项。就像给出的堆栈跟踪中的这一行一样:

10-08 20:45:14.421: E/AndroidRuntime(1024): at com.dev.app1234.Register.<init>(Register.java:68)