使用 JNI 创建、填充和返回 Java 类实例

Use JNI to Create, Populate and Return a Java Class Instance

提问by c12

I'm trying to use a JNI function to create a Java class and set some properties of that class using the DeviceId.java constructor method. I'm able to get the constructor method using the GetMethodID, but how would I create a new instance of Device.java and then set the properties (setId and setCache). The goal is to return a fully populated instance of Device.java Object to the caller. Any ideas?

我正在尝试使用 JNI 函数创建一个 Java 类并使用 DeviceId.java 构造函数方法设置该类的一些属性。我可以使用 GetMethodID 获取构造函数方法，但是如何创建 Device.java 的新实例，然后设置属性（setId 和 setCache）。目标是向调用者返回一个完全填充的 Device.java 对象实例。有任何想法吗？

JNI Function:

JNI 函数：

 JNIEXPORT jobject JNICALL Java_com_test_getID(JNIEnv *env, jclass cls) 
    {
        jmethodID cnstrctr;
        jclass c = (*env)->FindClass(env, "com/test/DeviceId");
        if (c == 0) {
            printf("Find Class Failed.\n");
         }else{
            printf("Found class.\n");
         }

        cnstrctr = (*env)->GetMethodID(env, c, "<init>", "(Ljava/lang/String;[B)V");
        if (cnstrctr == 0) {
            printf("Find method Failed.\n");
        }else {
            printf("Found method.\n");
        }

        return (*env)->NewObject(env, c, cnstrctr);
    }

Java Class:

Java类：

package com.test;

public class DeviceId {
    private String id;
    private byte[] cache;

        public DeviceId(){}
    public DeviceId(String id, byte[] cache){
        this.id=id;
        this.cache=cache;
    }

    public byte[] getCache() {
        return cache;
    }

    public void setCache(byte[] cache) {
        this.cache = cache;
    }

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }   
}

采纳答案by Edward Thomson

When you called GetMethodID, you provided the signature for the two-arg constructor. Thus, you just need to pass your jstringand a jbytearraywhen you call NewObject- for example:

当您调用 时GetMethodID，您提供了双参数构造函数的签名。因此，您只需要在调用时传递您的jstring和 a - 例如：jbytearrayNewObject

return (*env)->NewObject(env, c, cnstrctr, id, cache);

You don't need to call the setIdand setCachemethods unless you decide to call the 0-arg constructor - and that just complicates your code since you'll have to call GetMethodIDfor those and call them. Simpler to continue down the route you're on.

除非您决定调用 0-arg 构造函数，否则您不需要调用setId和setCache方法 - 这只会使您的代码复杂化，因为您必须GetMethodID调用它们并调用它们。更简单地继续沿着你所走的路线。

回答by kosiara - Bartosz Kosarzycki

I wanted to return a customJava object from JNI's cpp codeback to Java. The solution is to return a jobjectfrom cpp function and use our custom Java object in native method declaration.

我想将JNI 的 cpp 代码中的自定义Java 对象返回给 Java。解决方案是从 cpp 函数返回一个并在本地方法声明中使用我们自定义的 Java 对象。jobject

public class PyError {

    public String message;

    public boolean occurred;

    public PyError(boolean occurred, String message){
        this.message = message;
        this.occurred = occurred;
    }
} 

and method declaration in Java:

和 Java 中的方法声明：

native PyError nativePythonErrorOccurred();

on the cpp side:

在 cpp 方面：

extern "C" JNIEXPORT jobject JNICALL
Java_com_your_package_nativePythonErrorOccurred(JNIEnv *env, jobject obj) {

    jclass javaLocalClass = env->FindClass("com/your/package/PyError");

    if (javaLocalClass == NULL) {
        LOGP("Find Class Failed.\n");
    } else {
        LOGP("Found class.\n");
    }

    jclass javaGlobalClass = reinterpret_cast<jclass>(env->NewGlobalRef(javaLocalClass));

    // info: last argument is Java method signature
    jmethodID javaConstructor = env->GetMethodID(javaGlobalClass, "<init>", "(ZLjava/lang/String;)V");

    if (javaConstructor == NULL) {
        LOGP("Find method Failed.\n");
    } else {
        LOGP("Found method.\n");
    }

    jobject pyErrorObject = env->NewObject(javaGlobalClass, javaConstructor, true, env->NewStringUTF("Sample error body"));
    return pyErrorObject;
}

Determine the signature of the method using javap -s java.your.package.YourClass. Also, have a look here.

使用 确定方法的签名 javap -s java.your.package.YourClass。另外，看看这里。

If you encounter error similar to: JNI ERROR (app bug): attempt to use stale Global 0xf2ac01bayour method signature is wrong, you're passing wrong arguments to env->NewObject()or you're not using global state of jni objects - more here.

如果您遇到类似于以下JNI ERROR (app bug): attempt to use stale Global 0xf2ac01ba的错误：您的方法签名错误，您将错误的参数传递给env->NewObject()或您没有使用 jni 对象的全局状态 - 更多信息请点击此处。