Android Learning JNI, calling C in Java

First you must know what JNI is.

JNI (Java Native Interface) is the Java native Language interface, which provides a way to invoke the underlying language (c + +) in Java. In general, if the project must use the C language to operate the underlying hardware, then the JNI technique must be used.

So now that you're using Java to call C, you have to solve a few problems?

1:java is an object-oriented language, and C is a process-oriented language

The type of object,string in 2:java, the Boolean type, is not in C, how is the C language recognized?

In fact, these jni are done for us, in the JNI header file # include <jni.h> has

typedef unsigned char   Jboolean;       /* Unsigned 8 bits */typedef signed char     jbyte;          /* Signed 8 bits */typedef unsigned short  jchar;          /* unsigned bits */typedef short           jshort;         /* Signed-bits */typedef int             jint;           /* Signed */typedef long long       jlong;          /* Signed-bits */typedef float           jfloat;         /* 32-bit IEEE 754 */typedef double          jdouble;        /* 64-bit IEEE 754 */

/* * Reference types, in C. */typedef void* jobject;typedef jobject jclass;typedef         jobject         jstring;typedef Jobject         jarray;typedef jarray          jobjectarray;typedef jarray          jbooleanarray;typedef jarray          JbyteArray ; typedef jarray          jchararray;typedef jarray          jshortarray;typedef jarray jintarray;typedef          jarray          Jlongarray;typedef jarray          jfloatarray;typedef jarray          jdoublearray;typedef jobject jthrowable         ; typedef jobject         Jweak;

You can see that object objects in Java are in the void*, and this is known to everyone who has learned the C language.

Note: Before you learn JNI, you need to download the NDK installation on Google's website and configure the environment variables.

This time we learned how to invoke the C language using the Java language and then print Hello world!

First step: Build an Android Application project

Step two: Suggest a folder under the project root directory, named Jni

Step three: In the Jni folder, create a C file

Fourth step: In Java code, create a local method: HELLOFROMC

Fifth step: Implement the local function defined in step fourth in hello.c, but the function name must be: java+ Package name +java file + Local method name

such as: JAVA_COM_DEMO_HELLOWORLD_MAINACTIVITY_HELLOFROMC (jnienv* env, Jobject obj)

Sixth step: Convert C string to Java string return

Seventh Step: Create the Android.mk file in the Jni folder, similar to the makefile file

Eighth step: Execute the ndk-build.cmd instruction under the JNI folder

Nineth Step: Loading the dynamic library in Java, calling the local method

Tenth step: Deploy if the following error occurs

The CPU platform is a complement, the default is the arm of the architecture CPU, and I simulator with the X86, I would like to create a new arm simulator, not so troublesome

11th Step: Build a platform that supports X86 and create a new application.mk file under the Jni folder

Can be found to compile again, there is a x86 library

Deploy again to run:

You can see that the Hello_world returned by the C language can print correctly. Indicates that Java calls C language successfully.

Copyright NOTICE: This article for Bo Master original article, without Bo Master permission not reproduced.

Android Learning JNI, calling C in Java