Form Android SDK,It says about JNI like this:
JNI is the Java Native Interface. It defines a way for managed code (written in the Java programming language) to interact with native code (written in C/C++). It’s vendor-neutral, has support for loading code from dynamic shared libraries, and while cumbersome at times is reasonably efficient.
I am not too familiar with it, so I will read through the Java Native Interface Specification to get a sense for how JNI works and what features are available.
JavaVM and JNIEnv
JNI defines two key data structures, “JavaVM” and “JNIEnv”. Both of these are essentially pointers to pointers to function tables. (In the C++ version, they’re classes with a pointer to a function table and a member function for each JNI function that indirects through the table.) The JavaVM provides the “invocation interface” functions, which allow you to create and destroy a JavaVM. In theory you can have multiple JavaVMs per process, but Android only allows one.
The JNIEnv provides most of the JNI functions. Your native functions all receive a JNIEnv as the first argument.
The JNIEnv is used for thread-local storage. For this reason, you cannot share a JNIEnv between threads. If a piece of code has no other way to get its JNIEnv, you should share the JavaVM, and use GetEnv to discover the thread’s JNIEnv. (Assuming it has one; see AttachCurrentThread below.)
The C declarations of JNIEnv and JavaVM are different from the C++ declarations. The “jni.h” include file provides different typedefs depending on whether it’s included into C or C++. For this reason it’s a bad idea to include JNIEnv arguments in header files included by both languages. (Put another way: if your header file requires #ifdef __cplusplus, you may have to do some extra work if anything in that header refers to JNIEnv.)