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C呼叫java例子

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實現原理:使用JNI提供的介面來反射得到Java方法,進行呼叫。jni.h在JDK裡(jdk1.8.0_121/include/)。

(通過JNI可以實現java呼叫C,也可以實現C呼叫java)

 HelloWorld.java:

public class HelloWorld{
	public static void main(String[] args){
		System.out.println("Hello, World");
	}
	public static int square(int input){
		int output = input * input;
		return output;
	}
	public static int power(int input, int exponent){
		int output,i;
		output=1;
		for(i=0;i<exponent;i++){
			output *= input;
		}
		return output;
	}
}

hello_world.c:

#include<stdio.h>
#include<jni.h>

JNIEnv* create_vm(JavaVM **jvm)
{
    JNIEnv* env;
    JavaVMInitArgs args;
    JavaVMOption options;
    args.version = JNI_VERSION_1_6;
    args.nOptions = 1;
    options.optionString ="-Djava.class.path=./";
    args.options = &options;
    args.ignoreUnrecognized = 0;
    int rv;
    rv = JNI_CreateJavaVM(jvm,(void**)&env, &args);
    if (rv < 0 || !env)
        printf("Unable to Launch JVM%d\n",rv);
    else
        printf("Launched JVM! :)\n");
    return env;
}

 
void invoke_class(JNIEnv* env)
{
    jclass hello_world_class;
    jmethodID main_method;
    jmethodID square_method;
    jmethodID power_method;
    jint number=20;
    jint exponent=3;
    hello_world_class =(*env)->FindClass(env, "HelloWorld");
    main_method =(*env)->GetStaticMethodID(env, hello_world_class, "main","([Ljava/lang/String;)V");
    square_method =(*env)->GetStaticMethodID(env, hello_world_class, "square","(I)I");
    power_method =(*env)->GetStaticMethodID(env, hello_world_class, "power","(II)I");
    (*env)->CallStaticVoidMethod(env,hello_world_class, main_method, NULL);
    printf("%d squared is %d\n",number,
        (*env)->CallStaticIntMethod(env,hello_world_class, square_method, number));
    printf("%d raised to the %d power is%d\n", number, exponent,
        (*env)->CallStaticIntMethod(env,hello_world_class, power_method, number, exponent));
}


int main(int argc,char **argv)
{
    JavaVM *jvm;
    JNIEnv *env;
    env = create_vm(&jvm);
    if(env == NULL)
        return 1;
    invoke_class(env);
    return 0;
}

 # 編譯java

javac HelloWorld.java

 # 編譯C

gcc -D__int64="long long" -Ic:/java/jdk1.8.0_121/include/ -Ic:/java/jdk1.8.0_121/include/win32 -ohello_world hello_world.c -L c:/java/jdk1.8.0_121/jre/bin/server -ljvm

要點

a)     使用64位的JDK需要新增-D__int64="longlong"

b)     通過-I引數指定jni.h、jni_md.h的路徑。

c)     通過-L指定jvm.dll的路徑。

這3個檔案都在JDK資料夾裡面,可以通過搜尋找到具體的路徑。

#執行程式

hello_world

如果報找不到jvm.dll的錯,把jvm.dll(路徑:jdk1.8.0_121\jre\bin\server)註冊到系統。

可以寫Makefile一步到位,Makefile:

all: run
 
HelloWorld.class: HelloWorld.java
       javac HelloWorld.java

hello_world: hello_world.c
       gcc -D__int64="long long" -Ic:/java/jdk1.8.0_121/include/-Ic:/java/jdk1.8.0_121/include/win32 -o hello_world hello_world.c -Lc:/java/jdk1.8.0_121/jre/bin/server -ljvm

run: HelloWorld.class hello_world
       ./hello_world

clean:
       rm -f HelloWorld.class hello_world

 編譯和執行:

>make

輸出結果:

javac HelloWorld.java
gcc -Ic:/java/jdk1.8.0_121/include/ -Ic:/java/jdk1.8.0_121/include/win32 -ohello_world hello_world.c -L c:/java/jdk1.8.0_121/jre/bin/server -ljvm
./hello_world
Launched JVM! :)
Hello, World
20 squared is 400
20 raised to the 3power is 8000