[android help] Mismatch Dep Signature after Modifications on Android framework

I modified some code in PackageManagerService.java in Android framework(Android 4.2). After a make, and start the emulator, it report the following error:

10-05 08:20:34.213: I/dalvikvm(575): DexOpt: mismatch dep signature for '/system/framework/core.odex'
10-05 08:20:34.213: E/dalvikvm(575): /system/app/PackageInstaller.apk odex has stale dependencies
10-05 08:20:34.213: E/dalvikvm(575): odex source not available -- failing
10-05 08:20:34.223: W/PackageManager(575): StaleDexCacheError when reading apk: /system/app/PackageInstaller.apk
10-05 08:20:34.223: W/PackageManager(575): dalvik.system.StaleDexCacheError: /system/app/PackageInstaller.apk
10-05 08:20:34.223: W/PackageManager(575):  at dalvik.system.DexFile.isDexOptNeeded(Native Method)
10-05 08:20:34.223: W/PackageManager(575):  at com.android.server.pm.PackageManagerService.performDexOptLI(PackageManagerService.java:3375)
10-05 08:20:34.223: W/PackageManager(575):  at com.android.server.pm.PackageManagerService.scanPackageLI(PackageManagerService.java:3960)
10-05 08:20:34.223: W/PackageManager(575):  at com.android.server.pm.PackageManagerService.scanPackageLI(PackageManagerService.java:3238)
10-05 08:20:34.223: W/PackageManager(575):  at com.android.server.pm.PackageManagerService.scanDirLI(PackageManagerService.java:3017)
10-05 08:20:34.223: W/PackageManager(575):  at com.android.server.pm.PackageManagerService.(PackageManagerService.java:1058)
10-05 08:20:34.223: W/PackageManager(575):  at com.android.server.pm.PackageManagerService.main(PackageManagerService.java:837)
10-05 08:20:34.223: W/PackageManager(575):  at com.android.server.ServerThread.run(SystemServer.java:177)

If I clean and then make the full project again, it works ok. But it will report the same error after another modification. Since making a full build again will last a very long time, I prefer other solution.

I noticed the code in DexPrepare.cpp:

if (memcmp(signature, ptr, kSHA1DigestLen) != 0) {  
    LOGI("DexOpt: mismatch dep signature for '%s'", cacheFileName);  
    goto bail;  
}  

If I remove these lines of code, it will work. I used to work on Android 2.3, and there are still a signature checking in Android 2.3(Gingerbread), but after modifications on Gingerbread's framework, it will never report such errors. Why is that?

.

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