Brief introduction to JNI

1. Let’s talk
about JNI
Yongqiang Li

2. Agenda
 JNI Overview
 JNI Sample Step by Step
 More Details of JNI
 JVM Native Memory
 Reference

3. JNI (Java Native Interface)
Overview
Linux C/C++ Side
Java Side
Windows C/C++ Side
(Code, VM)
Mac OS C/C++ Side

4. Java –verbose:jni HelloWorld

5. JNI Sample (Step 1)
 Define
the Java class
and native functions

6. Step 2
 Use “javah” to
generate the header
for native functions.

7. Header File

8. Step 3
 Implement .c/.cpp

9. Step 4
 Build the native code
 g++ -I/usr/lib/java/include –
I/usr/lib/java/include/linux -share
org_yli_jni_SampleJNI.cpp -o libNative.so

10. Step 5
 Run it!
 java -Djava.library.path=. SampleJNI

11. A Real Case – Use Java to
start GTK+ application.

14. More Details of JNI
 JNI Type

16. JNIEnv
 Definea set of functions to manipulate
JNI data, get field/method id, find class.
 env->GetVersion()
 env->Throw()
 env->FindClass()
 env->NewLocalRef()
a Thread Local Storage variable

18. Fields
 Can’t access
directly
 Need to get Field
ID first

21. Methods
 Method ID is
needed, just like
fields.

22. Use “javap” to find out the signature.

25. And Constructor
 Use GetMethodID as well
 But method name is special. It’s “<init>”.
 Still use “javap” to find out its signature.
 Need use env->NewObject() to create a
new instance.

26.  As well, jstring can’t be manipulated directly.
jstring
• Depends on JNIEnv Functions
• GetStringChars/GetStringUTFChars
• ReleaseStringChars/ReleaseStringUTFChars
• GetStringLength/GetStringUTFLength
• NewString/NewStringUTF

29. Other things depends on
JNIEnv
 Manage Reference
 Find Class
 Access Array
 Throw exception
…

30. Native Memory

32. Consume native memory

33. More topics…
 Launch JVM in C/C++
 Traps and pitfalls

34. Reference
 Java Native Interface Specification
 Thanks for the memory