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？
phpcn_u15822017-05-16 13:37:09
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.
伊谢尔伦2017-05-16 13:37:09
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
