豌豆

在上一章(《多语言编程之 C and Java：JNI 30 分钟入门》)中讲解了 JNI 的基本使用。这一章将讨论一些更加实用的用法：如何在 JNI 中访问 Java 对象的成员变量或静态变量，以及回调 Java 中的成员方法或静态方法。

1 JNI 访问 Java 成员变量

1.1 实例

Java 实现

package transvar;
public class TransVar{
    static{
	System.loadLibrary("hello");
    }

    //成员变量(字段)
    private int age = 88;
    private String name = "Jims";

    //静态成员变量
    private static int years = 20;

    private native void modifyInstanceVariable();  
}

这个 Java 类中包含 3 个私有成员变量，名为 age 的 int 型变量， 名为 name 的 String 型变量和一个名为 years 的 int 型静态变量。我们将在使用 C 来访问或修改这些成员变量。

C 实现

JNIEXPORT void JNICALL Java_transvar_TransVar_modifyInstanceVariable
  (JNIEnv *env, jobject obj)
{
    //获取 this object 的类
    jclass thisClass = (*env)->GetObjectClass(env,obj);
    if(thisClass == NULL)
    {
      printf("thisClass null\n");
    }

    //获取字段ID
    jfieldID fidAge = (*env)->GetFieldID(env, thisClass, "age",  "I");
    jfieldID fidName = (*env)->GetFieldID(env, thisClass, "name", "Ljava/lang/String;");
    jfieldID fidYear = (*env)->GetStaticFieldID(env, thisClass, "years", "I");

    //获取字段
    jint age = (*env)->GetIntField(env, obj, fidAge);
    jint years = (*env)->GetStaticIntField(env, obj, fidYear);
    jstring name = (*env)->GetObjectField(env, obj, fidName);
    const char* pszName = (*env)->GetStringUTFChars(env, name, NULL);

    printf("[%d Years Later for  %s] \n", years, pszName);

    (*env)->ReleaseStringUTFChars(env, name, pszName);

    age += years;

    //修改成员变量
    (*env)->SetIntField(env, obj, fidAge, age);
}

1.2 访问 Java 变量

jfieldID GetFieldID(JNIEnv *env, jclass clazz, const char *name, const char *sig);
jfieldID GetStaticFieldID(JNIEnv *env, jclass clazz, const char *name, const char *sig);

此函数可以通过字段名和字段标识(signature)获取类或对象的字段 ID.

注 ：数组和引用类型的对象(Object)，都是以分号(;)结尾。对于嵌套类，使用 $ 符号来表示。

Get<type>Field()/Set<type>Field() 有一组函数，用于从对象中获取字段值或给变量设置值。

<type> Get<type>Field(JNIEnv *env, jobject obj, jfieldID fieldID);

jmethodID GetMethodID(JNIEnv *env, jclass cls, const char *name, const char *sig);

最后一个参数 sig 为方法的标识(signature)。对于这个标识，我们并不陌生，在上一章，我们使用 javah 命令生成的头文件中，对方法有一个名为 Signature 的注释，就是这个标识。 方法标识以括号 “(” 开始，一对括号内部为方法参数的标识，括号后面跟返回值的标识。如方法

void  MethedName(int a, int b);

的标识为： (II)V 。
除了参考 .h 文件的注释，还可以使用 Java 工具 javap 来查看方法标识。

> javap -s -p <JavaClassName>

在得到 MethodID 后，即可以通过如下函数来调用 Java 成员方法或表态方法：

<type> Call<type>Method(JNIEnv *env, jobject obj, jmethodID methodID, ...);
<type> Call<type>MethodA(JNIEnv *env, jobject obj, jmethodID methodID, const jvalue *args);
<type> Call<type>MethodV(JNIEnv *env, jobject obj, jmethodID methodID, va_list args);

<type> CallStatic<type>Method(JNIEnv *env, jclass clazz, jmethodID methodID, ...);
<type> CallStatic<type>MethodA(JNIEnv *env, jclass clazz, jmethodID methodID, const jvalue *args);
<type> CallStatic<type>MethodV(JNIEnv *env, jclass clazz, jmethodID methodID, va_list args);

3 JNI 创建 Java 对象

jclass FindClass(JNIEnv *env, const char *name);

jobject NewObject(JNIEnv *env, jclass cls, jmethodID methodID, ...);
jobject NewObjectA(JNIEnv *env, jclass cls, jmethodID methodID, const jvalue *args);
jobject NewObjectV(JNIEnv *env, jclass cls, jmethodID methodID, va_list args);
   // Constructs a new Java object. The method ID indicates which constructor method to invoke

jobject AllocObject(JNIEnv *env, jclass cls);
  // Allocates a new Java object without invoking any of the constructors for the object.

此外，还有如下函数可以创建 Java 数组对象：

jobjectArray NewObjectArray(JNIEnv *env, jsize length, jclass elementClass, jobject initialElement);
   // Constructs a new array holding objects in class elementClass.
   // All elements are initially set to initialElement.

jobject GetObjectArrayElement(JNIEnv *env, jobjectArray array, jsize index);
   // Returns an element of an Object array.

void SetObjectArrayElement(JNIEnv *env, jobjectArray array, jsize index, jobject value);
   // Sets an element of an Object array.

3.2 示例

public class Person{
    public Person(){};
    public Person(String name , int age){
	_name = name;
	_age = age;
    }
    private int _age;
    private String _name;  
}

并有 native 方法，将 name 和 age 传入 JNI 中，用于实例化一个 Person 对象并传回 Java:

private native Person createPerson(String name, int age);

则在 native 实现中，有如下过程：

JNIEXPORT jobject JNICALL Java_package_class_createPerson(JNIEnv *env, jobject obj, jstring name, jint age)
{
    jclass personClass = (*env)->FindClass(env, "packagename/Person");
    if(personClass == NULL)
    {
        printf("can not find class : packagename.Person");
        return NULL;
    }

    jmethodID midpersonInit = (*env)->GetMethodID(env, personClass, "<init>", "(Ljava/lang/String;I)V");
    if(midpersonInit == NULL)
    {
        printf("can not get methodID : <init>(Ljava/lang/String;I)V");
        return NULL;
    }
    jobject newPerson = (*env)->NewObject(env, personClass, midpersonInit, name, age);

    return newPerson;
}

4 附录

package transvar;
public class TransVar{
    static{
        System.loadLibrary("hello");
    }

    //成员变量(字段)
    private int age = 8;
    private String name = "Jims";

    //静态成员变量
    private static int years = 20;

    public int progress(int pro){
        System.out.println("[... " + pro + " YEARS LATER ...]");
        return 1;
    }

    private native void modifyInstanceVariable();

    private native Person createPerson(String name, int age);

    //For test
    public static void main(String args[]){
        TransVar tv = new TransVar();
        System.out.println(tv.name + " is  in the age of " + tv.age);
        tv.modifyInstanceVariable();
        System.out.println(tv.name + " is  in the age of " + tv.age);

        Person pjimSon = tv.createPerson("Jimson",1); 
        if(pjimSon != null){
           System.out.println("The person: " + pjimSon.getName() + " create from JNI in the age of " + pjimSon.getAge());
        }
    }    
}

package transvar;
public class Person{
    public Person(){};
    public Person(String name , int age){
        _name = name;
        _age = age;
    }
    private int _age;
    private String _name;  

    public int getAge(){return _age;}
    public String getName(){return _name;};
}

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

JNIEXPORT void JNICALL Java_transvar_TransVar_modifyInstanceVariable
  (JNIEnv *env, jobject obj)
{
    //获取 this object 的类
    jclass thisClass = (*env)->GetObjectClass(env,obj);
    if(thisClass == NULL)
    {
      printf("thisClass null\n");
    }

    //获取字段ID
    jfieldID fidAge = (*env)->GetFieldID(env, thisClass, "age",  "I");
    jfieldID fidName = (*env)->GetFieldID(env, thisClass, "name", "Ljava/lang/String;");
    jfieldID fidYear = (*env)->GetStaticFieldID(env, thisClass, "years", "I");

    //获取字段
    jint age = (*env)->GetIntField(env, obj, fidAge);
    jint years = (*env)->GetStaticIntField(env, obj, fidYear);
    jstring name = (*env)->GetObjectField(env, obj, fidName);
    const char* pszName = (*env)->GetStringUTFChars(env, name, NULL);

    printf("[... Time flies for %s ...] \n", pszName);

    (*env)->ReleaseStringUTFChars(env, name, pszName);

    jmethodID mid = (*env)->GetMethodID(env, thisClass, "progress", "(I)I");

    jint y = 0; 
    for(; y< years; y+=5)
    {
        jint i = (*env)->CallIntMethod(env,obj,mid,y);
        if(i != 1) break;
    }

    age += y;

    //修改成员变量
    (*env)->SetIntField(env, obj, fidAge, age);
}

JNIEXPORT jobject JNICALL Java_transvar_TransVar_createPerson(JNIEnv *env, jobject obj, jstring name, jint age)
{
    jclass personClass = (*env)->FindClass(env, "transvar/Person");
    if(personClass == NULL)
    {
        printf("can not find class : transvar.Person");
        return NULL;
    }

    jmethodID midpersonInit = (*env)->GetMethodID(env, personClass, "<init>", "(Ljava/lang/String;I)V");
    if(midpersonInit == NULL)
    {
        printf("can not get methodID : <init>(Ljava/lang/String;I)V");
        return NULL;
    }
    jobject newPerson = (*env)->NewObject(env, personClass, midpersonInit, name, age);

    return newPerson;
}