Android中，intent初始化为什么不能写在方法外面呢？

Question

初学者。今天写一个活动之间跳转的代码，用到了显式的intent，但是我在类中定义了一个intent的成员变量，在方法中使用时候，程序却崩溃了，这是为什么呢？

public class MusicPlay extends Activity{
    //下面这一句初始化出了错误
    public Intent intent=intent=new Intent(this,MusicServer.class);
    ....

如上代码，但是错误信息提示是：

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference

据说是在“空对象引用”上调用getPackageName()虚方法出现了错误。

起初，我以为是不是this是空从而导致问题，修改了代码：

public class MusicPlay extends Activity{
    Intent intent;
    public MusicPlay(){
        super();
        if(this!=null){
            intent=new Intent(this,MusicServer.class);
        }
    }
    ....

但是仍然出现了错误，错误信息还是一样的。通过条件判断我知道this不为空，那么为什么还说是空引用呢？

Answer 1

这么做之前, 先要了解清楚Activity与Context之间的关系: 虽然Activity继承了Context, 但是它却不是真正的实现类, 真正的实现可能是ContextWrapper#getBaseContext()返回对象所对应的类.

Activity是ContextWrapper的子类, 所以我们先找到并打开ContextWrapper.java源码, 关键代码如下:

public class ContextWrapper extends Context {
    public ContextWrapper(Context base) {
        mBase = base;
    }
    
    /**
     * Set the base context for this ContextWrapper.  All calls will then be
     * delegated to the base context.  Throws
     * IllegalStateException if a base context has already been set.
     * 
     * @param base The new base context for this wrapper.
     */
    protected void attachBaseContext(Context base) {
        if (mBase != null) {
            throw new IllegalStateException("Base context already set");
        }
        mBase = base;
    }
    
    ... ...
    
    /**
     * @return the base context as set by the constructor or setBaseContext
     */
    public Context getBaseContext() {
        return mBase;
    }

    @Override
    public AssetManager getAssets() {
        return mBase.getAssets();
    }

    @Override
    public Resources getResources() {
        return mBase.getResources();
    }

    ... ...

ContextWrapper里基于Context的调用都是直接使用mBase来间接实现调用的. 那么这个mBase是什么时候被赋值的呢? 找到并打开ActivityThread.java, 就能找到它被赋值的代码部分, 关键代码如下:

private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
    // System.out.println("##### [" + System.currentTimeMillis() + "] ActivityThread.performLaunchActivity(" + r + ")");

    ... ...

    // -------------------------------------------------------------------
    // 创建Activity实例
    // -------------------------------------------------------------------
    Activity activity = null;
    try {
        java.lang.ClassLoader cl = r.packageInfo.getClassLoader();
        activity = mInstrumentation.newActivity(
                cl, component.getClassName(), r.intent);
        StrictMode.incrementExpectedActivityCount(activity.getClass());
        r.intent.setExtrasClassLoader(cl);
        r.intent.prepareToEnterProcess();
        if (r.state != null) {
            r.state.setClassLoader(cl);
        }
    } catch (Exception e) {
        if (!mInstrumentation.onException(activity, e)) {
            throw new RuntimeException(
                "Unable to instantiate activity " + component
                + ": " + e.toString(), e);
        }
    }

    try {
        Application app = r.packageInfo.makeApplication(false, mInstrumentation);

        if (localLOGV) Slog.v(TAG, "Performing launch of " + r);
        if (localLOGV) Slog.v(
                TAG, r + ": app=" + app
                + ", appName=" + app.getPackageName()
                + ", pkg=" + r.packageInfo.getPackageName()
                + ", comp=" + r.intent.getComponent().toShortString()
                + ", dir=" + r.packageInfo.getAppDir());

        if (activity != null) {
            Context appContext = createBaseContextForActivity(r, activity);
            CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());
            Configuration config = new Configuration(mCompatConfiguration);
            if (r.overrideConfig != null) {
                config.updateFrom(r.overrideConfig);
            }
            if (DEBUG_CONFIGURATION) Slog.v(TAG, "Launching activity "
                    + r.activityInfo.name + " with config " + config);
            Window window = null;
            if (r.mPendingRemoveWindow != null && r.mPreserveWindow) {
                window = r.mPendingRemoveWindow;
                r.mPendingRemoveWindow = null;
                r.mPendingRemoveWindowManager = null;
            }
            
            // -------------------------------------------------------------------
            // 设置 appContext 为Activity 的 BaseContext
            // -------------------------------------------------------------------
            activity.attach(appContext, this, getInstrumentation(), r.token,
                    r.ident, app, r.intent, r.activityInfo, title, r.parent,
                    r.embeddedID, r.lastNonConfigurationInstances, config,
                    r.referrer, r.voiceInteractor, window);

以上, 可知: 实例化Activity时, ContextWrapper#getBaseContext()返回的是null, 因此, 不能在构造函数或者构造成员变量时直接调用与Context相关的任何函数和类的实例化. 如果需要, 则在其生命周期函数中去调用.

Answer 2

你的类继承了Activity类,那么它就有生命周期,所有逻辑都在这几个生命周期里面进行,换而言之,你的逻辑代码都要写在那几个生命周期的方法里面.一般来说,都是重写onCreate的方法,在那里面写页面跳转.你的方法的标识符命名也不规范

Answer 3

要重写onCreate()方法啊