异常截获: NoClassDefFoundError——How to Include A Jar File

前几天遇到这样一个问题：

我明明引入了一个jar包，ctrul+单击也能进入源码，却总在new类的时候（如：User user = new User()），出现NoClassDefFoundError，无法实例化类。

控制台打印堆栈如下：

11-15 16:41:00.495: E/AndroidRuntime(7757): FATAL EXCEPTION: main
11-15 16:41:00.495: E/AndroidRuntime(7757): java.lang.NoClassDefFoundError: com/google/zxing/integration/android/IntentIntegrator
11-15 16:41:00.495: E/AndroidRuntime(7757): 	at com.google.zxing.client.androidtest.ZXingTestActivity$3.onClick(ZXingTestActivity.java:139)
11-15 16:41:00.495: E/AndroidRuntime(7757): 	at android.view.View.performClick(View.java:4084)
11-15 16:41:00.495: E/AndroidRuntime(7757): 	at android.view.View$PerformClick.run(View.java:16966)
11-15 16:41:00.495: E/AndroidRuntime(7757): 	at android.os.Handler.handleCallback(Handler.java:615)
11-15 16:41:00.495: E/AndroidRuntime(7757): 	at android.os.Handler.dispatchMessage(Handler.java:92)
11-15 16:41:00.495: E/AndroidRuntime(7757): 	at android.os.Looper.loop(Looper.java:137)
11-15 16:41:00.495: E/AndroidRuntime(7757): 	at android.app.ActivityThread.main(ActivityThread.java:4928)
11-15 16:41:00.495: E/AndroidRuntime(7757): 	at java.lang.reflect.Method.invokeNative(Native Method)
11-15 16:41:00.495: E/AndroidRuntime(7757): 	at java.lang.reflect.Method.invoke(Method.java:511)
11-15 16:41:00.495: E/AndroidRuntime(7757): 	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:791)
11-15 16:41:00.495: E/AndroidRuntime(7757): 	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:558)
11-15 16:41:00.495: E/AndroidRuntime(7757): 	at dalvik.system.NativeStart.main(Native Method)
11-15 16:41:00.495: E/AndroidRuntime(7757): Caused by: java.lang.ClassNotFoundException: com.google.zxing.integration.android.IntentIntegrator
11-15 16:41:00.495: E/AndroidRuntime(7757): 	at dalvik.system.BaseDexClassLoader.findClass(BaseDexClassLoader.java:61)
11-15 16:41:00.495: E/AndroidRuntime(7757): 	at java.lang.ClassLoader.loadClass(ClassLoader.java:501)
11-15 16:41:00.495: E/AndroidRuntime(7757): 	at java.lang.ClassLoader.loadClass(ClassLoader.java:461)
11-15 16:41:00.495: E/AndroidRuntime(7757): 	... 12 more

很明显，这是没有找到相关类。当我们用到相关类，却没有其jar包的时候，就会报NoClassDefFoundError（can't find class from external jar）。
可是，我明明引入了啊，也add到Build Path里面了，为什么还会报NoClassDefFoundError呢？

是我引入jar包的方法不对，放的位置不对，还是？

那么，如何引入一个jar文件呢（how to include a jar file）?

take the following steps, then you won't get the error:

1.Create a folder in the root of your project called libs.

2.Add the jar to that folder.

3.Right-click the jar and click to add to build path.

4.Clean your project.

5.Build project.

6.Run.

其实，在android project里面，在ADT的作用下，引入jar包，只需要将jar包拷入工程的libs目录下，不需要再add to build path，系统会自动将libs下的jar包添加到build path（实际上是添加到Android
Dependencies这个library下）

哎，规范编程，“约定优于配置”，再也不会偷懒，把jar文件随便放了。