有时候，自己下载的或者拷贝过来的JNI项目出现莫名错误，通常是找不到头文件，可能解决方案如下：
Removing the C nature:
The only way I could find to reliably removed the C nature from the project was by hand editing Eclipse‘s .project file for the project.
- Close the Eclipse project (e.g. by quitting Eclipse).
- Open the
.projectfile in a text or xml editor. There will be at least 2<buildCommand>nodes that need to be removed. Remove the<buildCommand>node with nameorg.eclipse.cdt.managedbuilder.core.genmakebuilderand all its children, and the<buildCommand>node with nameorg.eclipse.cdt.managedbuilder.core.ScannerConfigBuilderand its children. Finally, remove the lines:<nature>org.eclipse.cdt.core.cnature</nature><nature>org.eclipse.cdt.core.ccnature</nature><nature>org.eclipse.cdt.managedbuilder.core.managedBuildNature</nature><nature>org.eclipse.cdt.managedbuilder.core.ScannerConfigNature</nature> - Completely remove the
.cprojectfile.
Adding back the Android Native nature
Reopen the project in Eclipse. Then right-click on the project in the Project Explorer, and from the "Android Tools" contextual menu, choose "Add Native Support...".
参考：http://stackoverflow.com/questions/23122934/eclipse-adt-unresolved-inclusion-jni-h
Android JNI项目提示“Unresolved inclusion: <jni.h>”可能的其他解决方案：
- Paths and Symbols中路径不正确，考虑修改Android.MK文件内容，比如增加空行等（注意Android文件中不需要有LOCAL_C_INCLUDES字段 ，系统会根据LOCAL_SRC_FILES自动添加需要的头文件，否则头文件将出现二义性），重新编译一次项目，Android.MK文件中LOCAL_SRC_FILES所依赖的头文件将会在Project Properties -> C/C++ General -> Paths and Symbols中被自动添加；
- string.h、jni.h等C头文件“Unresolved inclusion”，考虑： NDK Project -> New -> Folder -> Advanced -> Link to alternate location(Linked Folder)，添加：D:\ADT\android-ndk-r9d\platforms\android-18\arch-arm\usr\include；
- 考虑：properties--->C/C++ General，关闭Code Analysis功能，解决大部分错误；