java.lang.NoClassDefFoundError: com.google.gson.Gson

异常如下：
03-03 07:53:37.044: W/dalvikvm(2150): VFY: unable to find class referenced in signature (Lcom/google/gson/Gson;)
03-03 07:53:37.044: E/dalvikvm(2150): Could not find class 'com.google.gson.Gson', referenced from method com.***.MainActivity.onCreate
03-03 07:53:37.044: W/dalvikvm(2150): VFY: unable to resolve new-instance 50 (Lcom/google/gson/Gson;) in Lcom/***/MainActivity;
03-03 07:53:37.044: D/dalvikvm(2150): VFY: replacing opcode 0x22 at 0x0015
03-03 07:53:37.044: D/dalvikvm(2150): DexOpt: unable to opt direct call 0x0051 at 0x17 in Lcom/***/MainActivity;.onCreate
03-03 07:53:37.124: D/dalvikvm(2150): GC_FOR_ALLOC freed 69K, 5% free 2624K/2760K, paused 4ms, total 7ms
03-03 07:53:37.124: I/dalvikvm-heap(2150): Grow heap (frag case) to 3.695MB for 1127532-byte allocation
03-03 07:53:37.124: D/dalvikvm(2150): GC_FOR_ALLOC freed 2K, 4% free 3723K/3864K, paused 6ms, total 7ms
03-03 07:53:37.154: D/dalvikvm(2150): GC_CONCURRENT freed <1K, 4% free 3722K/3864K, paused 1ms+1ms, total 5ms
03-03 07:53:37.234: D/dalvikvm(2150): GC_CONCURRENT freed 63K, 4% free 4089K/4244K, paused 4ms+1ms, total 10ms
03-03 07:53:37.234: D/dalvikvm(2150): WAIT_FOR_CONCURRENT_GC blocked 6ms
03-03 07:53:37.294: D/dalvikvm(2150): GC_FOR_ALLOC freed 52K, 5% free 4436K/4648K, paused 4ms, total 45ms
03-03 07:53:37.294: D/dalvikvm(2150): GC_FOR_ALLOC freed 49K, 5% free 4787K/5000K, paused 4ms, total 4ms
03-03 07:53:37.304: D/AndroidRuntime(2150): Shutting down VM
03-03 07:53:37.304: W/dalvikvm(2150): threadid=1: thread exiting with uncaught exception (group=0xaa53e908)
03-03 07:53:37.304: E/AndroidRuntime(2150): FATAL EXCEPTION: main
03-03 07:53:37.304: E/AndroidRuntime(2150): java.lang.NoClassDefFoundError: com.google.gson.Gson
03-03 07:53:37.304: E/AndroidRuntime(2150):     at com.***.MainActivity.onCreate(MainActivity.java:73)
03-03 07:53:37.304: E/AndroidRuntime(2150):     at android.app.Activity.performCreate(Activity.java:5104)
03-03 07:53:37.304: E/AndroidRuntime(2150):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
03-03 07:53:37.304: E/AndroidRuntime(2150):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
03-03 07:53:37.304: E/AndroidRuntime(2150):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
03-03 07:53:37.304: E/AndroidRuntime(2150):     at android.app.ActivityThread.access$600(ActivityThread.java:141)
03-03 07:53:37.304: E/AndroidRuntime(2150):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
03-03 07:53:37.304: E/AndroidRuntime(2150):     at android.os.Handler.dispatchMessage(Handler.java:99)
03-03 07:53:37.304: E/AndroidRuntime(2150):     at android.os.Looper.loop(Looper.java:137)
03-03 07:53:37.304: E/AndroidRuntime(2150):     at android.app.ActivityThread.main(ActivityThread.java:5041)
03-03 07:53:37.304: E/AndroidRuntime(2150):     at java.lang.reflect.Method.invokeNative(Native Method)
03-03 07:53:37.304: E/AndroidRuntime(2150):     at java.lang.reflect.Method.invoke(Method.java:511)
03-03 07:53:37.304: E/AndroidRuntime(2150):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
03-03 07:53:37.304: E/AndroidRuntime(2150):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
03-03 07:53:37.304: E/AndroidRuntime(2150):     at dalvik.system.NativeStart.main(Native Method)
 

相关环境：Eclipse IDE for Java Developers Version: 2018-12 (4.10.0)

                  Android 4.2.2

                  Gson 2.6.1

解决办法：将jar包存放目录错误，改为 libs 。