JNI教程（五）

JNI操作字符串简单的例子

一、编写StringObjectJava.java文件

class StringValue {        private String m_value;        private int m_byteLen;        StringValue() {                m_value = "empty";                m_byteLen = m_value.getBytes().length;        }        public String getValue() {                return m_value;        }        public void setValue(String value) {                m_value = value;                m_byteLen = value.getBytes().length;        }        int getByteLength() {                 return m_byteLen;        }}public class StringObjectJava{    //所有native关键词修饰的都是对本地的声明    public native void setStringJni(StringValue obj);    static {        System.loadLibrary("test");//载入本地库    }    public static void main(String[] args) {       StringValue string_value_obj = new StringValue();       StringObjectJava obj = new StringObjectJava();       System.out.println("setStringJni before  str = " + string_value_obj.getValue() + "\t bytelen = " + string_value_obj.getByteLength());       obj.setStringJni(string_value_obj);       System.out.println("setStringJni after  str = " + string_value_obj.getValue() + "\t bytelen = " + string_value_obj.getByteLength());    }}

二、执行javac命令
javac StringObjectJava.java

三、执行javah命令创建JNI头文件
(1) javah StringObjectJava
(2) mv StringObjectJava.h StringObjectJni.h

四、创建JNI实现文件
vi StringObjectJni.cpp

#include "StringObjectJni.h"#include "iostream"/* class:     StringObjectJava * Method:    setStringJni * Signature: (LStringValue;)V */JNIEXPORT void JNICALL Java_StringObjectJava_setStringJni  (JNIEnv *env, jobject static_instance, jobject string_value_obj) {        std::cout << "hello Java_StringObjectJava_setStringJni implements C++"<< std::endl;        jclass cls            = env->GetObjectClass(string_value_obj);        jmethodID function_id = env->GetMethodID(cls , "getValue" , "()Ljava/lang/String;");        jstring input_js = (jstring)env->CallObjectMethod(string_value_obj, function_id);        function_id = env->GetMethodID(cls , "getByteLength" , "()I");        jint input_len = env->CallIntMethod(string_value_obj,function_id);        const char* input_ptr = env->GetStringUTFChars(input_js,NULL);        std::cout << "java string_value_obj data :" << std::string(input_ptr,input_len)<<std::endl;        // Return value        std::string ret_val("hello world jni");        jstring ret_js_val = env->NewStringUTF(ret_val.c_str());        function_id = env->GetMethodID(cls , "setValue" , "(Ljava/lang/String;)V");        env->CallVoidMethod(string_value_obj, function_id,ret_js_val);        // 调用GetStringUTFChars函数必须释放资源，否则会内存泄漏。        env->ReleaseStringUTFChars(input_js,input_ptr);        // 调用NewStringUTF函数必须释放资源，否则内存泄漏。        env->DeleteLocalRef(ret_js_val);}

五、编译动态链接库
(1) g++ -fPIC -I/usr/lib/jvm/java-1.6.0-openjdk-1.6.0.0.x86_64/include/ -I/usr/lib/jvm/java-1.6.0-openjdk-1.6.0.0.x86_64/include/Linux -c StringObjectJni.cpp

(2) g++ -shared -o libtest.so StringObjectJni.o

六、执行java类
java -Djava.library.path=. StringObjectJava