.通过JNI加载java虚拟机

前面已经分析了java命令加载java虚拟机的方法，即通过JNI和虚拟机交互。因此我们可以很容易的自己写相应的启动代码，实现在C++程序中执行java程序，调用java类的方法。

     要测试的Java类(Test.java)的代码是：

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public class Test {  
    public Test(){  
        System.out.println("Test's constructor");  
    }  
 
    public void sayHello(){  
        System.out.println("Test object say : Hello!");  
    }  
 
    public static int add(int arg1,int arg2){  
        return arg1 + arg2;  
    }  
      
    public static void main(String[] args) {  
        for (int i = 0; i < args.length; i++) {  
            System.out.println(args[i]);  
        }  
    }  
} 
public class Test {
    public Test(){
        System.out.println("Test's constructor");
    }

    public void sayHello(){
        System.out.println("Test object say : Hello!");
    }

    public static int add(int arg1,int arg2){
        return arg1 + arg2;
    }
   
    public static void main(String[] args) {
        for (int i = 0; i < args.length; i++) {
            System.out.println(args[i]);
        }
    }
}




   加载jvm的C++(StartJVM.cpp)代码如下：

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/* 
* startjvm.cpp 
* 
*  Created on: 2010-1-28 
*      Author: Xiutao Zang 
*/ 
#include <iostream>  
#include <jni.h>  
 
using namespace std;  
 
int main()  
{  
    JavaVMOption options[1];  
    options[0].optionString = "-Djava.class.path=.";  
    JavaVMInitArgs args;  
    memset(&args, 0, sizeof(args));  
    args.version = JNI_VERSION_1_2;  
    args.nOptions = 1;  
    args.options = options;  
    args.ignoreUnrecognized = JNI_FALSE;  
 
    JNIEnv *env = 0;  
    JavaVM *vm = 0;  
 
    jint result = JNI_CreateJavaVM(&vm, (void **)&env, &args);  
 
    if (result == JNI_OK)  
    {  
        cout << "create the jvm successfully!" << endl;  
        jclass mainClass = env->FindClass("Test");  
        if (mainClass != 0)  
        {  
            cout << "find the main class successfully!" << endl;  
 
            jclass stringClass = env->FindClass("java/lang/String");  
            enum 
            {  
                LENGTH = 2  
            };  
            const char *mainParams[LENGTH] = { "Hello ", "World!" };  
            jstring jstringTemp;  
            jobjectArray mainArgs = env->NewObjectArray(LENGTH, stringClass, 0);  
            for (int i = 0; i < LENGTH; i++)  
            {  
                jstringTemp = env->NewStringUTF(mainParams[i]);  
                env->SetObjectArrayElement(mainArgs, i, jstringTemp);  
            }  
 
            jmethodID mainID = env->GetStaticMethodID(mainClass, "main","([Ljava/lang/String;)V");  
            if (mainID != 0)  
            {  
                cout << "get main method successfully and invoke it!" << endl;  
                env->CallStaticVoidMethod(mainClass, mainID, mainArgs);  
            }  
            else 
            {  
                cout << "get main method fail!" << endl;  
            }  
 
            jmethodID methodID = env->GetStaticMethodID(mainClass, "add", "(II)I");  
            if(methodID !=0){  
                int sum = env->CallStaticIntMethod(mainClass,methodID,4,6);  
                cout << "the result of Test.add(4,6) is : " << sum << endl;  
               }  
 
            methodID = env->GetMethodID(mainClass,"<init>","()V");  
            if(methodID != 0){  
                    jobject res = env->NewObject(mainClass,methodID);  
                    jmethodID objectMethodID = env->GetMethodID(mainClass,"sayHello","()V");  
                    if(objectMethodID != 0){  
                        env->CallVoidMethod(res,objectMethodID);  
                    }  
            }  
        }  
        else 
        {  
            cout << "can't find main class!" << endl;  
        }  
 
        cout << "destroy the jvm!" << endl;  
        vm->DestroyJavaVM();  
        return 0;  
    }  
    else 
    {  
        printf("create java jvm fail!\n");  
        return -1;  
    }  
} 
/*
* startjvm.cpp
*
*  Created on: 2010-1-28
*      Author: Xiutao Zang
*/
#include <iostream>
#include <jni.h>

using namespace std;

int main()
{
JavaVMOption options[1];
options[0].optionString = "-Djava.class.path=.";
JavaVMInitArgs args;
memset(&args, 0, sizeof(args));
args.version = JNI_VERSION_1_2;
args.nOptions = 1;
args.options = options;
args.ignoreUnrecognized = JNI_FALSE;

JNIEnv *env = 0;
JavaVM *vm = 0;

jint result = JNI_CreateJavaVM(&vm, (void **)&env, &args);

if (result == JNI_OK)
{
cout << "create the jvm successfully!" << endl;
jclass mainClass = env->FindClass("Test");
if (mainClass != 0)
{
cout << "find the main class successfully!" << endl;

jclass stringClass = env->FindClass("java/lang/String");
enum
{
LENGTH = 2
};
const char *mainParams[LENGTH] = { "Hello ", "World!" };
jstring jstringTemp;
jobjectArray mainArgs = env->NewObjectArray(LENGTH, stringClass, 0);
for (int i = 0; i < LENGTH; i++)
{
jstringTemp = env->NewStringUTF(mainParams[i]);
env->SetObjectArrayElement(mainArgs, i, jstringTemp);
}

jmethodID mainID = env->GetStaticMethodID(mainClass, "main","([Ljava/lang/String;)V");
if (mainID != 0)
{
cout << "get main method successfully and invoke it!" << endl;
env->CallStaticVoidMethod(mainClass, mainID, mainArgs);
}
else
{
cout << "get main method fail!" << endl;
}

jmethodID methodID = env->GetStaticMethodID(mainClass, "add", "(II)I");
if(methodID !=0){
    int sum = env->CallStaticIntMethod(mainClass,methodID,4,6);
    cout << "the result of Test.add(4,6) is : " << sum << endl;
   }

methodID = env->GetMethodID(mainClass,"<init>","()V");
if(methodID != 0){
jobject res = env->NewObject(mainClass,methodID);
jmethodID objectMethodID = env->GetMethodID(mainClass,"sayHello","()V");
if(objectMethodID != 0){
env->CallVoidMethod(res,objectMethodID);
}
}
}
else
{
cout << "can't find main class!" << endl;
}

cout << "destroy the jvm!" << endl;
vm->DestroyJavaVM();
return 0;
}
else
{
printf("create java jvm fail!\n");
return -1;
}
}





        我的运行环境是CentOS，AMD64平台。

相关环境变量：

export JAVA_HOME=/root/DevTools/jdk1.6.0_16
export JRE_HOME=$JAVA_HOME/jre
export CLASSPATH=$CLASSPATH:.:$JAVA_HOME/lib:$JRE_HOME/lib
export PATH=$JAVA_HOME/bin:$JRE_HOME/bin:$PATH

export LD_LIBRARY_PATH=$JRE_HOME/lib/amd64/server:$JRE_HOME/lib/amd64



编译命令：

g++ -o StartJVM StartJVM.cpp -I${JAVA_HOME}/include -I${JAVA_HOME}/include/linux -L${JRE_HOME}/lib/amd64/server -ljvm



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