Java Native Interface - Lecture Slides | CMSC 212, Study notes of Computer Science

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CMSC 212 – S05 (lect 26)^1

Announcements

l Program

l Final Exam

• Thursday, Dec 15
• in H.J. Patterson 0226.
• 4:00-6:00 pm

l Reading

• http://java.sun.com/docs/books/tutorial/native1.1/

l Office Hours

• TA office hours are through Wednesday, December 14
• My office hours tomorrow
  • 10:00-11:
  • If PG schools are delayed or closed – cancelled
• My office hours next week
  • Regular through Tuesday
  • I’ll post extra office hours for Wed and Thurs
  • None on Friday

CMSC 212 – S05 (lect 26)^2

Java Native Interface

l Goals (combine code from different languages in both

directions)

• Sometimes need to access things in lower level language
• Provide full access to Java Objects in other languages

l Different Reasons for Both Directions

• When would you want to use “native methods” from Java Code?
• When would you want to use Java code from a native application?

l Keys Ideas

• Need to identify signatures of native methods
• Need to incorporate native code into program

CMSC 212 – S05 (lect 26)^3

Process of Compiling Using JNI

Helloworld.java Java Compiler (javac)

gcc with dynamic lib flags

Helloworld.class

Helloworld.h

javah –jni

stdio.h

jni.h Hello.so

Write Java code

java HelloWorld

Hello World!!

Write C code

Helloworld.c

CMSC 212 – S05 (lect 26)^4

Using JNI Functions

l Declaring Functions in Java

• (where the function to be used is written in C)
• Declare like a normal method, but add “native” keyword
• public native void displayHelloWorld();

l Loading the Code

• Use loadLibrary call
• Making it static forces it to happen automatically
• System.loadLibrary("hello")
• Java’s equivalent of dlopen
• adds the appropriate name completion for this system

l Invoke Native Method

• new HelloWorld().displayHelloWorld();
• this invokes the C code

CMSC 212 – S05 (lect 26)^5

C Function Signature-Constructing the function name

• Java_ + Full Classname + _ + Method Name JNIEXPORT void JNICALL Java_HelloWorld_displayHelloWorld (JNIEnv *, jobject);

#include <jni.h> #include "HelloWorld.h" #include <stdio.h>

JNIEXPORT void JNICALL Java_HelloWorld_displayHelloWorld (JNIEnv *env, jobject obj) { printf("Hello world!\n"); return; }

class HelloWorld { public native void displayHelloWorld();

static { System.loadLibrary("hello"); }

public static void main(String[] args) { new HelloWorld().displayHelloWorld(); } }

CMSC 212 – S05 (lect 26)^6

Accessing Java Data from C

l Mapping Types

Long Jlong 64

Int Jint 32

Short Jshort 16

Char Jchar 16

Boolean Jboolean 8, unsigned

Byte Jbyte 8

Java Type Native Type Size (bits)

CMSC 212 – S05 (lect 26)^7

Java Object Types

jobject

jstring java.lang.String objects jthrowable java.lang.Throwable exceptions

jarray (jintArray, jfloatArray…)

jclass java.lang.Class objects

CMSC 212 – S05 (lect 26)^8

Access Java Arrays

l Need to call special functions to get data and size

l Example:

JNIEXPORT jfloat JNICALL Java_FloatArray_sumArray(JNIEnv *env, jobject obj, jfloatArray arr) { jfloat *body, sum = 0; jsize I, len;

len = (env)->GetArrayLength(env, arr); body = (env)->GetFloatArrayElements(env, arr, 0); for (i=0; i<len; i++) { sum += body[i]; } (*env)->ReleaseFloatArrayElements(env, arr, body, 0); return sum; }

CMSC 212 – S05 (lect 26)^9

Calling Java Methods from C

l Steps

• Get Object handle
• Get MethodID – need to know signature
  • (argument-types)return-type
• Call method

l Example

JNIEXPORT void JNICALL Java_Callbacks_nativeMethod( JNIEnv env, jobject obj, jint depth) { jclass cls = (env)->GetObjectClass(env, obj); jmethodID mid = (env)->GetMethodID(env, cls, "callback", "(I)V"); if (mid == 0) { return; } printf("In C, depth = %d, about to enter Java\n", depth); (env)->CallVoidMethod(env, obj, mid, depth); printf("In C, depth = %d, back from Java\n", depth); }

CMSC 212 – S05 (lect 26)^10

Accessing Object Fields

l Getting FieldID

• Need to get a unique id for the field
• Must specify type information
• fid = (*env)->GetFieldID(env, cls, "s", "Ljava/lang/String;");
  • cls is of type jclass
  • s is field name
• Can make one call to GetFieldID
  • Really computes offset for item

l Getting the data

• Pass the fid, env, and object
• str = (*env)->GetObjectField(env, obj, fid);