JNI技术学习（二）：Java 与 C++ 之间传递String，自定义类，自定义类的数组

小灰灰 2021-09-22 14:12 867阅读 0赞

博主在实习中遇到了实际开发的问题，关于JNI，之前已经有一个博客和小的demo：  
https://blog.csdn.net/Applying/article/details/81572167

现在的需求是：JAVA端获取到一定格式的数据，并传递给C++端，C++端获取这部分数据之后，进行算法的操作，将操作结果重新以另外特定的结构格式传递给JAVA端使用。

思路：其实我们现在要实现的，就是JAVA与C++之间交互特定的数据结构，这里常见碰到的有三种：

*  String类
 *  JAVA类
 *  类数组

接下来，分3个部分，简单写一下demo：

## 1. JAVA与C++之间传递String ##

### JAVA端代码： ###

### C++端代码： ###

// Impl.cpp:
    JNIEXPORT jstring JNICALL Java_aa_bb_cc_Hello_testGetString
      (JNIEnv * env, jobject)
    { 
    	// 1. 创建字符串
    	char * a = "hello java";
    
    	// 2. 直接转为jstring类型
    	jstring returnValue = env->NewStringUTF(a);
    	return returnValue;
    }
    
    JNIEXPORT void JNICALL Java_aa_bb_cc_Hello_testPassString
      (JNIEnv * env, jobject, jstring string)
    { 
    	const char * a = env->GetStringUTFChars(string, NULL);
    	cout << a << endl;
    	return;
    }

### Junit测试代码及结果： ###

// testHello.java:
    class HelloTest { 
    	private Hello hello = new Hello();
    	@Test
    	void testGetString() { 
    		System.loadLibrary("HelloC");
    		String temp = hello.testGetString();
    		System.out.println(temp);
    	}
    	
    	@Test
    	void testPassString() { 
    		System.loadLibrary("HelloC");
    		hello.testPassString("hello ");
    	}
    }

![这里写图片描述][70]  
![这里写图片描述][70 1]

## 2. JAVA与C++之间传递JAVA类 ##

### JAVA端代码： ###

// Hello.java:
    public class Hello { 
    	static { 
    		System.loadLibrary("HelloC");
    	}
    	public native Student testGetStudent();
    	public native void testPassStudent(Student student);
    }
    
    // 这里定义了一个学生类，Student.java:
    public class Student { 
    	private int height;
    	private int weight;
    	private int[] grade;
    	
    	public Student(int height, int weight, int[] grade) { 
    		super();
    		this.height = height;
    		this.weight = weight;
    		this.grade = grade;
    	}
    	
    	public Student() { 
    	}
    	
    	@Override
    	public String toString() { 
    		return "Student [height=" + height + ", weight=" + weight + ", 
    		grade=" + Arrays.toString(grade) + "]";
    	}
    }

### C++端代码： ###

// Impl.cpp:
    struct Student{ 
    	int height;
    	int weight;
    	int grade[2];
    };
    
    JNIEXPORT jobject JNICALL Java_aa_bb_cc_Hello_testGetStudent
      (JNIEnv * env, jobject)
    { 
    	// 1. 模拟原C++代码中的结构体
    	Student student;
    	student.height = 166;
    	student.weight = 60;
    	student.grade[0] = 100;
    	student.grade[1] = 100;
    
    	// 2. 获得对应JAVA类的各个属性
    	// 这里的“Laa/bb/cc/Student;”，去通过javah指令生成的头文件里即可找到
    	jclass objClass = env->FindClass("Laa/bb/cc/Student;");
    	jfieldID heightFieldId = env->GetFieldID(objClass, "height", "I");
    	jfieldID weightFieldId = env->GetFieldID(objClass, "weight", "I");
    	// “[” 代表一个数组结构，“I”代表的是Int
    	jfieldID gradeFieldId = env->GetFieldID(objClass, "grade", "[I");
    	
    	// 3. 创建新的对象并将结构体中数据赋值给JAVA对象
    	jobject object = env->AllocObject(objClass);
    	const jint array[2] = { student.grade[0], student.grade[1]};
    	jintArray array_ = env->NewIntArray(2);
    	env->SetIntArrayRegion(array_, 0, 2, array);
    	env->SetIntField(object, heightFieldId, student.height);
    	env->SetIntField(object, weightFieldId, student.weight);
    	// 这里关于这个数组转换，我不太清楚是否有更简单的方法
    	env->SetObjectField(object, gradeFieldId, array_);
    	return object;
    }
    
    JNIEXPORT void JNICALL Java_aa_bb_cc_Hello_testPassStudent
      (JNIEnv *env, jobject, jobject object)
    { 
    	// 1. 获得对应JAVA类的各个属性
    	jclass jclass = env->FindClass("Laa/bb/cc/Student;");
    	jfieldID heightFieldId = env->GetFieldID(jclass, "height", "I");
    	jfieldID weightFieldId = env->GetFieldID(jclass, "weight", "I");
    	jfieldID gradeFieldId = env->GetFieldID(jclass, "grade", "[I");
    
    	// 2. 获得传入类的具体属性值
    	jint height = env->GetIntField(object, heightFieldId);
    	jint weight = env->GetIntField(object, weightFieldId);
    	jintArray grade = (jintArray)env->GetObjectField(object, gradeFieldId);
    	jint* array = env->GetIntArrayElements(grade, NULL);
    
    	// 3. 将具体的属性值再封装成C++中的结构体
    	// 这里直接打印测试函数传参是否成功
    	cout << "height: " << height << " weight: " << weight << endl
    		<< "grade[0]: " << array[0] << " grade[1]: " << array[1] <<endl;
    	return;
    }

### Junit测试代码及结果： ###

// testHello.java:
    class HelloTest { 
    	private Hello hello = new Hello();
    	@Test
    	void testGetStudent() { 
    		System.loadLibrary("HelloC");
    		Student student = hello.testGetStudent();
    		System.out.println(student.toString());
    	}
    	
    	@Test
    	void testPassStudent() { 
    		System.loadLibrary("HelloC");
    		Student student = new Student(171, 67, new int[] { 99, 100});
    		hello.testPassStudent(student);
    	}
    }

![这里写图片描述][70 2]  
![这里写图片描述][70 3]

## 3. JAVA与C++之间传递类数组 ##

### JAVA端代码： ###

// Hello.java:
    public class Hello { 
    	static { 
    		System.loadLibrary("HelloC");
    	}
    	public native Student[] testGetStudentArray();
    	public native void testPassStudentArray(Student[] students);
    }

### C++端代码： ###

// Impl.cpp:
    struct Student{ 
    	int height;
    	int weight;
    	int grade[2];
    };
    
    JNIEXPORT jobjectArray JNICALL Java_aa_bb_cc_Hello_testGetStudentArray
      (JNIEnv * env, jobject)
    { 
    	const int number = 2;
    	// 1. 模拟数据
    	Student a[number];
    	a[0].height = 180;  a[0].weight = 100;  
    	a[0].grade[0] = 100;  a[0].grade[1] = 99;
    	a[1].height = 181;  a[1].weight = 101;
    	a[1].grade[0] = 88;	 a[1].grade[1] = 88;
    
    	// 2. 获得JAVA类的各个属性
    	jclass jclass = env->FindClass("Laa/bb/cc/Student;");
    	jfieldID heightFieldId = env->GetFieldID(jclass, "height", "I");
    	jfieldID weightFieldId = env->GetFieldID(jclass, "weight", "I");
    	jfieldID gradeFieldId = env->GetFieldID(jclass, "grade", "[I");
    	// 这里获得构造方法
    	jmethodID initMethodId = env->GetMethodID(jclass, "<init>", "()V");
    	
    	// 3. 将数据封装成对象再分装成数组
    	jobject object;
    	jobjectArray array = env->NewObjectArray(number, jclass, NULL);
    	for (int i = 0; i < number; i++) { 
    		object = env->NewObject(jclass, initMethodId);
    		env->SetIntField(object, heightFieldId, a[i].height);
    		env->SetIntField(object, weightFieldId, a[i].weight);
    		jintArray tempArray = env->NewIntArray(number);
    		const jint array_[2] = { a[i].grade[0], a[i].grade[1]};
    		env->SetIntArrayRegion(tempArray, 0, 2, array_); 
    		env->SetObjectField(object, gradeFieldId, tempArray);
    		// 将对象插入数组的特定位置
    		env->SetObjectArrayElement(array, i, object);
    	}
    	return array;
    }
    
    JNIEXPORT void JNICALL Java_aa_bb_cc_Hello_testPassStudentArray
      (JNIEnv *env, jobject, jobjectArray array)
    { 
    	// 1. 获得对应JAVA类的各个属性
    	jclass jclass = env->FindClass("Laa/bb/cc/Student;");
    	jfieldID heightFieldId = env->GetFieldID(jclass, "height", "I");
    	jfieldID weightFieldId = env->GetFieldID(jclass, "weight", "I");
    	jfieldID gradeFieldId = env->GetFieldID(jclass, "grade", "[I");
    	int length = env->GetArrayLength(array);
    
    	// 2. 将数组拆分，每个对象转为结构体
    	jobject obj;
    	Student stu;
    	for (int i = 0; i < length; i++) { 
    		obj = env->GetObjectArrayElement(array, i);
    		stu.height = env->GetIntField(obj, heightFieldId);
    		stu.weight = env->GetIntField(obj, weightFieldId);
    		jintArray temp = (jintArray)env->GetObjectField(obj, gradeFieldId);
    		env->GetIntArrayRegion(temp, 0, env->GetArrayLength(temp), (jint*)stu.grade);
    	    // 这里用打印测试结果替代转为结构体 
    		cout << "height: " << stu.height << " weight: " << stu.weight << endl
    			<< "grade[0]: " << stu.grade[0] << " grade[1]: " << stu.grade[1] <<endl;
    	}
    
    	return;
    }

### Junit测试代码及结果： ###

// testHello.java:
    class HelloTest { 
    	private Hello hello = new Hello();
    	
    	@Test
    	void testGetStudentArray() { 
    		System.loadLibrary("HelloC");
    		Student[] students = hello.testGetStudentArray();
    		for (Student student : students) { 
    			System.out.println(student.toString());
    		}	
    	}
    	
    	@Test
    	void testPassStudentArray() { 
    		System.loadLibrary("HelloC");
    		ArrayList<Student> students = new ArrayList<>();
    		students.add(new Student(171, 67, new int[] { 100, 99}));
    		students.add(new Student(155, 45, new int[] { 100, 99}));
    		Student[] stu = new Student[students.size()];
    		students.toArray(stu);
    		hello.testPassStudentArray(stu);
    	}
    }

![这里写图片描述][70 4]  
![这里写图片描述][70 5]

## 4. 一些遇到的坑&建议 ##

### ① 自定义的类一定要有无参的构造函数 ###

这里如果没有无参的构造函数，在执行的时候，会出现：  
![这里写图片描述][70 6]

### ② 将java.library.path指定为动态链路库的生成路径 ###

我们在VS处生成动态链路库文件xx.dll，将编辑器的java.library.path路径指定为对应的生成路径，可以减少生成之后移动到java.library.path的麻烦。关于设置java.library.path的路径：  
eclipse可以参考：里面有提到如何设置  
https://blog.csdn.net/Applying/article/details/81572167  
对于IDEA：RUN-》Edit Configurations：  
![这里写图片描述][70 7]  
在VM options处，配置：`-Djava.library.path=xxx` 即可

关于一些函数签名表or参数类型后期再整理一波  
这个只是一个实现的demo，记录下来，方便忘记时查看

[70]: /images/20210920/eb0195573fa143799752501e7088ca78.png
[70 1]: /images/20210920/a06ba6dffe674d9187331d2bfa3c9a03.png
[70 2]: /images/20210920/9e1bcec5689a4810b7488ed42b77392c.png
[70 3]: /images/20210920/d1dcb4c1cbeb453aaa00246d457db074.png
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[70 5]: /images/20210920/5d7137e49e724ddc90d09c19baafb2a0.png
[70 6]: /images/20210920/6aef952ab90d4a0694cb6a042480c9f8.png
[70 7]: /images/20210920/0342ae70ffe0489895b5ce3da42dcfcf.png