JNI学习笔记，C语言调用Java

public class Hello { 
    private String name;    /*C程序中调用SetObjectField()来设置*/ 
    private int age;        /*C程序中调用SetIntField()来设置*/ 
     
    public static void main(String args[]) { 
        System.out.println("Hello, world!"); 
    } 
 
    public int sayhello_to(String name) { 
        System.out.println("Hello, "+name+"! I am "+this.name+", "+age+" years old.");        
        return 123; 
    } 
 
    public static void sayhello_to() { 
    } 
}

#include <stdio.h>   
#include <jni.h>  
 
 
jint create_vm(JavaVM** jvm, JNIEnv** env)  
{   
    JavaVMInitArgs args;   
    JavaVMOption options[1];   
    args.version = JNI_VERSION_1_6;  /*创建1.6版本的java虚拟机*/ 
    args.nOptions = 1;   
    options[0].optionString = "-Djava.class.path=./"; /*在当前目录下查找类*/ 
    args.options = options;   
    args.ignoreUnrecognized = JNI_FALSE;   
    return JNI_CreateJavaVM(jvm, (void **)env, &args);   
}   
 
 
int main(int argc, char **argv) 
{ 
    JavaVM* jvm; 
    JNIEnv* env; 
 
    jclass cls; 
    int ret = 0; 
 
    jmethodID mid; 
    jmethodID cid; 
 
    jobject jobj; 
    jstring jstr; 
 
    jfieldID nameID; 
    jfieldID ageID; 
 
    int r; 
         
    /* 1. create java virtual machine */ 
    if (create_vm(&jvm, &env)) { 
        printf("can not create jvm\n"); 
        return -1; 
    } 
 
    /* 2. get class */ 
    cls = (*env)->FindClass(env, "Hello"); /*获取Hello类对象*/ 
    if (cls == NULL) { 
        printf("can not find hello class\n"); 
        ret = -1; 
        goto destroy; 
    } 
 
    /* 3. create object  
     * 3.1 get constructor method 
     * 3.2 create parameters 
     * 3.3 NewObject 
     * 注意：调用类的静态成员方法的时候不需要实例化对象 
     */ 
 
    /*<init>表示获取构造方法，构造函数的签名可以用javap -p -s Hello.class查看*/ 
    /* Get the method ID for the String constructor */ 
    cid = (*env)->GetMethodID(env, cls,    "<init>", "()V"); 
    if (cid == NULL) { 
        ret = -1; 
        printf("can not get constructor method"); 
        goto destroy; 
    } 
 
    /*调用类的构造函数实例化一个对象*/ 
    jobj = (*env)->NewObject(env, cls, cid); 
    if (jobj == NULL) { 
        ret = -1; 
        printf("can not create object"); 
        goto destroy; 
    } 
 
    /* get/set field 
     * 1. get field id 
     * 2. get/set field 
     */ 
     
    /*设置Hello类对象的name属性，arg4是属性的签名*/ 
    nameID = (*env)->GetFieldID(env, cls, "name", "Ljava/lang/String;"); 
    if (nameID == NULL) { 
        ret = -1; 
        printf("can not get field name"); 
        goto destroy; 
    } 
    jstr = (*env)->NewStringUTF(env, "Bill"); 
    (*env)->SetObjectField(env, jobj, nameID, jstr); /*将name属性设置为“Bill”*/ 
 
    /*设置Hello类对象的age属性*/ 
    ageID = (*env)->GetFieldID(env, cls, "age", "I"); 
    if (ageID == NULL) { 
        ret = -1; 
        printf("can not get field age"); 
        goto destroy; 
    } 
    (*env)->SetIntField(env, jobj, ageID, 10); 
 
    /* 4. call method 
     * 4.1 get method 
     * 4.2 create parameter 
     * 4.3 call method 
     */ 
 
    /*调用Java程序中的“sayhello_to”成员方法id*/ 
    mid = (*env)->GetMethodID(env, cls, "sayhello_to","(Ljava/lang/String;)I"); 
    if (mid == NULL) { 
        ret = -1; 
        printf("can not get method\n"); 
        goto destroy; 
    } 
 
    jstr = (*env)->NewStringUTF(env, "abcd@qq.com"); 
 
    /*调用Java程序中的“sayhello_to”成员方法并获取返回值。 
     * 对比静态方法: 
     * 这里是CallIntMethod()且其参数是对象 
     * eg: static void main()是调用CallStaticVoidMethod()且参数是类 
     */ 
    r = (*env)->CallIntMethod(env, jobj, mid, jstr); 
    /*打印Java程序中的“sayhello_to”成员方法的返回值*/ 
    printf("ret = %d\n", r); 
 
destroy: 
 
    (*jvm)->DestroyJavaVM(jvm); 
    return ret; 
}

javac Hello.java 
 
javap -p -s Hello.class //获取成员函数和成员属性的Signature，在C中调用Java函数或设置类的属性的时候使用。 
 
gcc -I/usr/lib/jvm/java-1.7.0-openjdk-amd64/include/ -o caller caller.c -L /usr/lib/jvm/java-7-openjdk-amd64/jre/lib/amd64/server -ljvm 
 
export LD_LIBRARY_PATH=/usr/lib/jvm/java-7-openjdk-amd64/jre/lib/amd64/server 
./caller

运行结果：

$ ./caller
Hello, abcd@qq.com! I am Bill, 10 years old.
ret = 123