2019-09-06：什么是JNI？具体说说如何实现Java与C++的互调？
Moosphan opened this issue · comments
PS：可通过代码示例来说明Java与C++的调用过程。
我是来看答案的。
The Java Native Interface (JNI) is a standard to integrate in a portable way C++ and Java code. It works in both directions: you can call a C++ library from Java or you can call Java components from C++. In this tutorial, I'll explain the second approach.
How does this work?
-
We first have to find the right class with FindClass(), which acts as a class loader. It will search for an appropriate .class file in the list of directories that was provided at JVM initialization. If the Java class is included in a package, you shall provide its full name.
-
The class is then passed to GetStaticMethod(), which shall find the right method in the class. The last parameter of this function is the most difficult one: the method signature. In case of any mismatch here, the method won't be found. "()" means a function with no parameter and "V" that the return type is void.
-
A static method is independent of any object. So we can then call the method with CallStaticVoidMethod().
1： java 调用 c++ 方法， 通过 java类定义 native 方法， 使用javah 生成.h 头文件，定义cpp文件实现该方法， java 层就可以直接调用该方法了。
2： c++ 调用java 方法，分静态方法和非静态方法.
静态方法调用
JniHelper::callStaticStringMethod(classPath, "getAppName");
非静态方法调用
JniMethodInfo->CallVoidMethod(jobj,info.methodID,forceLogin);
- 在Java类中声明native方法
- 使用javac命令将Java类生成class文件
- 使用javah文件生成头文件
- 编写cpp文件实现jni方法
- 生成so库
到此为止，java就可以通过调用native方法调用c++函数了，对于在c++中调用java方法， - 通过完整类名获取jclass
- 根据方法签名和名称获取构造方法id
- 创建对象（如果要调用的是静态方法则不需要创建对象）
- 获取对象某方法id
- 通过JNIEnv根据返回值类型、是否是静态方法调用对应函数即可
