如何使用JNI从C中的jobject获取值？

Question

如何在C中获取Jobject的值？
我JNI在C中使用并调用了Java函数。该参数是一个jobject，它看起来应该像这样：{"John", "Ganso", 5}。
现在我想从该对象获取值，但我不知道如何。您对如何解决有什么建议？
我在C中的结构看起来像我在Java中的类。

我的代码如下所示：

JNIEXPORT void JNICALL
Java_model_JNIResultSet_printToFile(JNIEnv *env, jobject obj,
    jobject o) {

// How can I get values of jobject o?
}

Answer 1

/* PassObject.java */
package recipeNo020;

public class PassObject {

    /* This is the native method we want to call */
    public static native void displayObject(Object obj);

    /* Inside static block we will load shared library */
    static {
            System.loadLibrary("PassObject");
    }

    public static void main(String[] args) {
    /* This message will help you determine whether
        LD_LIBRARY_PATH is correctly set
    */
    System.out.println("library: "
        + System.getProperty("java.library.path"));

    /* Create object to pass */
    CustomClass cc = new CustomClass();
    cc.iVal = 1;
    cc.dVal = 1.1;
    cc.cVal = 'a';
    cc.bVal = true;
    cc.sVal = "Hello from the CustomClass";
    cc.oVal = new OtherClass();
    cc.oVal.sVal = "Hello from the OtherClass";

    /* Call to shared library */
        PassObject.displayObject(cc);
  }
}

/* CustomClass.java */
package recipeNo020;

public class CustomClass {
    public int iVal;
    public double dVal;
    public char cVal;
    public boolean bVal;
    public OtherClass oVal;
    public String sVal;
}

/* OtherClass.java */
package recipeNo020;

public class OtherClass {
    public String sVal;
}

/* recipeNo020_PassObject.c */

#include <stdio.h>
#include "jni.h"
#include "recipeNo020_PassObject.h"

JNIEXPORT void JNICALL Java_recipeNo020_PassObject_displayObject
  (JNIEnv *env, jclass obj, jobject objarg) {

    /* Get objarg's class - objarg is the one we pass from
       Java */
    jclass cls = (*env)->GetObjectClass(env, objarg);

    /* For accessing primitive types from class use
           following field descriptors

           +---+---------+
           | Z | boolean |
           | B | byte    |
           | C | char    |
           | S | short   |
           | I | int     |
           | J | long    |
           | F | float   |
           | D | double  |
           +-------------+
    */

    /* Get int field

       Take a look here, we are passing char* with
           field descriptor - e.g. "I" => int
        */
    jfieldID fidInt = (*env)->GetFieldID(env, cls, "iVal", "I");
    jint iVal = (*env)->GetIntField(env, objarg, fidInt);
    printf("iVal: %d\n", iVal);

    /* Get double field */
    jfieldID fidDouble = (*env)->GetFieldID(env, cls, "dVal", "D");
    jdouble dVal = (*env)->GetIntField(env, objarg, fidDouble);
    printf("dVal: %f\n", dVal);

    /* Get boolean field */
    jfieldID fidBoolean = (*env)->GetFieldID(env, cls, "bVal", "Z");
    jboolean bVal = (*env)->GetIntField(env, objarg, fidBoolean);
    printf("bVal: %d\n", bVal);

    /* Get character field */
    jfieldID fidChar = (*env)->GetFieldID(env, cls, "cVal", "C");
    jboolean cVal = (*env)->GetIntField(env, objarg, fidChar);
    printf("cVal: %c\n", cVal);

    /* Get String field */
    jfieldID fidString = (*env)->GetFieldID(env, cls, "sVal", "Ljava/lang/String;");
    jobject sVal = (*env)->GetObjectField(env, objarg, fidString);

    // we have to get string bytes into C string
    const char *c_str;
    c_str = (*env)->GetStringUTFChars(env, sVal, NULL);
    if(c_str == NULL) {
            return;
    }

    printf("sVal: %s\n", c_str);

    // after using it, remember to release the memory
    (*env)->ReleaseStringUTFChars(env, sVal, c_str);

    /* Get OtherClass */
    jfieldID fidOtherClass = (*env)->GetFieldID(env, cls, "oVal", "LrecipeNo020/OtherClass;");
    jobject oVal = (*env)->GetObjectField(env, objarg, fidOtherClass);

    jclass clsOtherClass = (*env)->GetObjectClass(env, oVal);

    /* Once we have OtherClass class and OtherClass object
       we can access OtherClass'es components
    */

    /* Get String field from OtherClass */
    jfieldID fidStringOtherClass = (*env)->GetFieldID(env, clsOtherClass, "sVal", "Ljava/lang/String;");
    jobject sValOtherClass = (*env)->GetObjectField(env, oVal, fidStringOtherClass);

    // we have to get string bytes into C string
    const char *c_str_oc;
    c_str_oc = (*env)->GetStringUTFChars(env, sValOtherClass, NULL);
    if(c_str_oc == NULL) {
        return;
    }

    printf("OtherClass.sVal: %s\n", c_str_oc);

    // after using it, remember to release the memory
    (*env)->ReleaseStringUTFChars(env, sValOtherClass, c_str_oc);
}

/* Make file */

include ../Makefile.common

all: compilejava compilec

compilec:
    cc -g -shared -fpic -I${JAVA_HOME}/include -I${JAVA_HOME}/include/$(ARCH) c/recipeNo020_PassObject.c -o lib/libPassObject.$(EXT)


compilejava:
    $(JAVA_HOME)/bin/javac -cp target -d target java/recipeNo020/OtherClass.java
    $(JAVA_HOME)/bin/javac -cp target -d target java/recipeNo020/CustomClass.java
    $(JAVA_HOME)/bin/javac -cp target -d target java/recipeNo020/PassObject.java
    $(JAVA_HOME)/bin/javah -jni -d c -cp target recipeNo020.PassObject

test:
    $(JAVA_HOME)/bin/java -Djava.library.path=$(LD_LIBRARY_PATH):./lib -cp target recipeNo020.PassObject

.PHONY: clean
clean:
    -rm -rfv target/*
    -rm c/recipeNo020_PassObject.h
    -rm -rfv lib/*

/* directory structure */
.
??? Makefile
??? c
?   ??? recipeNo020_PassObject.c
??? java
?   ??? recipeNo020
?       ??? CustomClass.java
?       ??? OtherClass.java
?       ??? PassObject.java
??? lib
??? target

/* execution */
make
make test

您会发现检出项目并进行编译更加容易：

https://github.com/mkowsiak/jnicookbook

Answer 2

无论您是编写本机方法还是在 C 程序中嵌入 JVM，您都应该阅读JNI 文档。它包含您需要了解的大量信息，包括用于访问 Java 对象字段的 JNI 函数的详细信息。

简而言之，要获取字段的值，您

1. 通过 获取其字段 ID GetFieldID()。这将要求您还获得（或已经拥有）一个jclass表示该字段所属类的对象；您可以通过GetObjectClass()或获得该信息FindClass()。

2. 使用适合字段声明类型的GetXXXField()方法之一获取字段的值。XXX对于 JavaString来说，那就是GetObjectField(); 对于 Javaint来说就是GetIntField().

如果您想查看字符串的详细信息，您还需要使用一些字符串操作函数，例如GetStringUTFChars()和GetStringUTFLength()。并且不要忽视这些以修改后的 UTF-8 进行操作的函数与以“Unicode 字符”（实际上意味着 UTF-16）进行操作的类似函数之间的重要区别。