Java JNI简介

- Connect Java and C++ Using JNI

整理电脑时发现之前写的一篇关于JNI的文档，放在以供参考。

Our (old) product uses JNI(Java Native Interface) to call C++ code from Java side.

JNI Basic

Using JNI, you only declare a method in Java, while implement that method in C++ Code. Follow below steps, so that when you invoke that Java method, Java/JVM will help invoke corresponding C++ implementation.

1. Declare method to be native in java

    public class ProxyService {
        // ...
        public native void execute(long cmdCode);
        //...
    }
   

2. Generate header files for C++
   Java provides command to help generate header files.

    $ javac com/abc/def/ghi/jkl/xyzserver/*.java # compile first  
    $ javah -jni com.abc.def.ghi.jkl.xyzserver.ProxyService 
   

   It generates a header file called com\_abc\_def\_ghi\_jkl\_xyzserver\_ProxyService.h. You can find below lines there,

    JNIEXPORT void JNICALL Java_com_abc_def_ghi_jkl_xyzserver_ProxyService_execute
      (JNIEnv *, jobject, jlong);
   

3. Implement that C++ function

    JNIEXPORT void JNICALL Java_com_abc_def_ghi_jkl_xyzserver_ProxyService_execute
      (JNIEnv * env, jobject instance, jlong commandCode)
    {
        // C++ code for your "native" java method
    }
   

   The above is a very simple example of JNI. Usually, you already have an existing C++ class, and just want to “wrap” the C++ class into a Java class. To do this, let Java object have a member of long type, which holds the address of a C++ object of your C++ class.

    public class ProxyService {
        long nativeInstanceAddr;
        public native void newCppObj();
        public native void delCppObj();
        public native void execute(long cmdCode);
    }
   

   In C++ code,

    JNIEXPORT void JNICALL Java_com_abc_def_ghi_jkl_xyzserver_ProxyService_newCppObj
      (JNIEnv * env, jobject instance)
    {
        jclass c = env->getobjectclass(obj);
        jfieldid field = env->getfieldid(c, "nativeInstanceAddr", "j");
        jlong address = env->getlongfield(obj, field);
        address = // new some c++ object
    }
   
    JNIEXPORT void JNICALL Java_com_abc_def_ghi_jkl_xyzserver_ProxyService_delCppObj
      (JNIEnv * env, jobject instance)
    {
        jlong address = // get the value of nativeInstanceAddr field like in newCppObj()
        delete (SomeCPPClass *)address;
    }
   
    JNIEXPORT void JNICALL Java_com_abc_def_ghi_jkl_xyzserver_ProxyService_execute
      (JNIEnv * env, jobject instance, jlong commandCode)
    {
        jlong address = // get the value of nativeInstanceAddr field like in newCppObj()
        (SomeCPPClass *)address->execute(commandCode);
    }
   

4. Load C++ library in Java

    public class DLLLoader {
        public synchronized static void loadDll() {
            //...
            System.loadLibrary(libName);
        }
    }
   

When to Release C++ Object

Java objects are taken care by JVM. However, for the C++ objects which are created via JNI, the JVM doesn’t manage (garbage-collect) them automatically (them live in their “own heap”). You have to delete C++ objects manually.

For example, C++ objects can be released when a transaction is committed or rolled back, or when a HTTP request finishes.

public void commit() {
    try
    {
        //...
    }
    catch(Exception e)
    {
        //...
    }
    finally
    {
        // delete c++ objects here
        // for example, call ProxyService::delCppObj() here
    }
}