Java4Android第10課_2：JNI--c調用java

在c/c++中，也可以調用java語言的方法和屬性。

分爲三種情況：調用靜態方法、調用非靜態方法、設置屬性

下面僅僅是列出調用java非靜態方法的一個例子，以供參考：

Hello.java：

class Hello{
int sayHello(String str)
{
System.out.println("Hello " + str);
return 325;
}
}

c_hello.c

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

jint create_vm(JavaVM** jvm, JNIEnv** env) 
{  
    JavaVMInitArgs args;  
    JavaVMOption options[1];  
    args.version = JNI_VERSION_1_6;  
    args.nOptions = 1;  
    options[0].optionString = "-Djava.class.path=./";  
    args.options = options;  
    args.ignoreUnrecognized = JNI_FALSE;  
    return JNI_CreateJavaVM(jvm, (void **)env, &args);  
}  

int main()
{
jint ret = 0;
JavaVM* jvm;
JNIEnv* env;
jclass cls;
jmethodID mid;
jmethodID cid;
jobject jobj;
jstring jstr;
jint retFrmJava;
/* 1. create java VM*/
ret = create_vm(&jvm, &env);
if(ret)
{
printf("can not create jvm\n");
return -1;
}

/* 2. find java class*/
cls = (*env)->FindClass(env, "Hello");
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 constructor method parameters
* 3.3 NewObject
*/
/* 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;
}
jobj = (*env)->NewObject(env, cls, cid);
if (jobj == NULL) {
ret = -1;
printf("can not create object");
goto destroy;
}

/* 4. call method
* 4.1 get method
* 4.2 create method parameter
* 4.3 call method
*/
mid = (*env)->GetMethodID(env, cls, "sayHello", "(Ljava/lang/String;)I");
if (mid == NULL) {
ret = -1;
printf("can not get method\n");
goto destroy;
}
jstr = (*env)->NewStringUTF(env, "lee");
retFrmJava = (*env)->CallIntMethod(env, jobj, mid, jstr);
printf("ret = %d\n", retFrmJava);
destroy:
(*jvm)->DestroyJavaVM(jvm);
return ret;
}

編譯：

javac Hello.java

gcc -I/usr/lib/jvm/java-1.7.0-openjdk-amd64/include/ -o c_hello c_hello.c -L /usr/lib/jvm/java-7-openjdk-amd64/jre/lib/amd64/server -ljvm

執行：

LD_LIBRARY_PATH=/usr/lib/jvm/java-7-openjdk-amd64/jre/lib/amd64/server ./c_hello