[Java JNI quick learning tutorial] and Java JNI quick learning

[Java JNI quick learning tutorial] and Java JNI quick learning

 1. Introduction to JNI

JNI is the abbreviation of Java Native Interface, which is a Java Local Interface.

Problem Source: because it is difficult to compile underlying applications in Java, it is difficult to be competent in some Java programs with high real-time requirements (the real-time requirements are not yet involved, and such topics need to be studied ).

Solution: Using JNI in Java, you can call an existing local library (C/C ++ develops any program and class libraries related to the system), which greatly facilitates Java development.

2. JNI quick learning tutorial

2.1 questions:

Use JNI to write a piece of code to implement the string_Java_Test_helloworld (JNIEnv * env, jclass cls, jstring j_str) function. Add hello to the string j_str ("world") and return the result.

2.2 Problem Solving Process:

I. Compile the Test. java class:

 1 public class Test{ 2     // native interface 3     public native String helloworld(String text); 4      5     public static void main(String[] args){ 6         // Load dynamic library 7         System.loadLibrary("Test2"); 8         Test ts = new Test(); 9         String text = ts.helloworld("world");10         System.out.println(text);11     }12 }

Note:

1. Load the dynamic Class Library: System. loadLibrary ("Test2"); [Test2.dll is loaded in Windows, Test2.so is loaded in Linux]

II. Compile the Test. java File

Enter cmd and enter the command> javac Test. java

III. Generate the Test. h file

Enter cmd and enter the command> javah Test

The content of the Test. h file is as follows:

 1 /* DO NOT EDIT THIS FILE - it is machine generated */ 2 #include <jni.h> 3 /* Header for class Test */ 4  5 #ifndef _Included_Test 6 #define _Included_Test 7 #ifdef __cplusplus 8 extern "C" { 9 #endif10 /*11  * Class:     Test12  * Method:    helloworld13  * Signature: (Ljava/lang/String;)Ljava/lang/String;14  */15 JNIEXPORT jstring JNICALL Java_Test_helloworld16   (JNIEnv *, jobject, jstring);17 18 #ifdef __cplusplus19 }20 #endif21 #endif

Note:

1. function declaration, fixed format: JNIEXPORT; return type: jstring; JNI call: JNICALL; Java _ complete name_method name: Java_Test_helloworld;

2. function parameters: Call the jni. h encapsulated function pointer: JNIEnv; Java class itself: jobject; input parameter of the Java file: jstring.

IV. Compile the C language file Test2.c to enable the Test class to call the dynamic link library function:

 1 #include "Test.h" 2 #include <string.h> 3  4 JNIEXPORT jstring JNICALL Java_Test_helloworld 5   (JNIEnv *env, jobject obj, jstring string){ 6     const char* str = (*env)->GetStringUTFChars(env,string,0); 7     char cap[128]; 8     cap[0] = 'h'; 9     cap[1] = 'e';10     cap[2] = 'l';11     cap[3] = 'l';12     cap[4] = 'o';13      14     strcat(cap,str);15       16     (*env)->ReleaseStringUTFChars(env,string,0);17     return (*env)->NewStringUTF(env,cap);18   }

Note:

1. Because Java uses double-byte characters, the C language itself is a single-byte character, you need to use (* env)-> GetStringUTFChars () to convert the string between Java and C;

2. GetStringUTFChars () and NewStringUTF (). The first is to convert UTF8 to the c encoding format, and the second is to return a UTF8 String Based on the C string;

3. ReleaseStringUTFChars () is used to release objects. There are virtual machines in Java for garbage collection, but these objects must be manually recycled in C language; otherwise, memory leakage may occur.

V. Compile and run

Compile:

Enter cmd and enter the command> gcc-I "D: \ Program Files \ Java \ jdk1.8.0 _ 45 \ include"-I "D: \ Program Files \ Java \ jdk1.8.0 _ 45 \ include \ win32 "-- share Test2.c-o Test2.dll

Run:

Enter cmd and enter the command> java Test

The running result is as follows:

Helloworld

3. Summary:

Step 1: Compile the Java class (Test. java) with the native method, and use the javac tool to compile the Java class (Generate Test. class );

Step 2: Use javah to generate the header file (Test. h) corresponding to the native method );

Step 3: Use C/C ++ to implement the corresponding header file (Test2.c) and compile it into a dynamic link library (Test2.so ).

Running Environment: Windows 64-bit operating system, JDK 1.8, mingw64 (GCC ).

4. Reference:

Http://www.cnblogs.com/icejoywoo/archive/2012/02/22/2363709.html

[Reprinted this article please note the original Address Source: http://www.cnblogs.com/eesijian/p/5843879.html]