jni debug in eclipse

A little example Java Native Interface usage in java and debugging jni library within eclipse Java Development Toolkit & C/C++ Development Toolkit. I use eclipse 3.3M5 and CDT 4.0M5.

I'll use make, gdb and gcc from cygwin.

1. There is JNIDebug java project, which contains java class Example:

   public class Example {
   
   static {
     System.loadLibrary("example");
   }
   
   private native int getValue();
   
   public static void main(String[] args) {
     final Example example = new Example();
     System.out.println("value is:" + example.getValue());
   }
   }
   

2. Create Managed Make C Project with project name JNI, select project type Cygwin DLL and Debug Configuration.
   
   
3. Write the following code to c/example.c:
   

   #include <jni.h>
   #include <stdio.h>
   
   #include "Example.h"
   
   
   /*
   * Class:     Example
   * Method:    getValue
   * Signature: ()I
   */
   JNIEXPORT jint JNICALL Java_Example_getValue
   (JNIEnv *env, jobject jobj)
   {
   printf("hello from jni/c!\n");
   return 1;
   }
   

   
   
4. Set the following properties of JNI project:
   
   • C/C++ Build
   • GCC C Compiler
   • Directories
   • add $(JAVA_HOME)/include and $(JAVA_HOME)/include/win32
     
   • Miscellaneous
   • other flags: -c -mno-cygwin
     
   • GCC C Linker
   • Miscellaneous
   • other flags: -mno-cygwin -Wl,--add-stdcall-alias
     
   • Shared Library Settings
   • Shared (-shared) should be switched on
   • Build settings
   • Artifact name: example
     
   • Artifact extension: dll
   
   so, have to Build project JNI.
   
5. Well, there are two projects: java project JNIDebug and managed make C project JNI, have the following structure:
   
   
   


6. Create debug entries in eclipse

Put a break point somewhere in native code.

Create java application debug launch configuration: choose project JNIDebug, main class Example and check on Stop in main


Start this debug configuration, vm will suspend on main().

When, create C/C++ Attach to Local Application debug configuration with name JNI: select temporary C/C++ project, select C/C++ Application native/example.dll through Browse option. In Debugger tab select Cygwin GDB Debugger - gdb or full path to gdb if it is not in the PATH environment variable,


7. Now start debug entry JNI.

After start debugging it is necessary to attach to java application process. It's necessary to remember eclipse javaw.exe pid before start application. It's not actually for eclipse >= 3.3 as it not start javaw.exe process. Select javaw.exe pid according to our application.

attaching to process

We'll stopped on some system dll position due to java application suspended on break point. Press Resume in gdb process
and when Resume in Java Application process.

The final :

Stepping over break point in example.c

alternative way with Makefile:

JAVAH=$(JAVA_HOME)/bin/javah
BUILD_PATH=build/classes

all: Example.h native/example.dll

Example.h: $(BUILD_PATH)/Example.class
$(JAVAH) -classpath $(BUILD_PATH) -d src/c Example

native/example.dll: src/c/example.c
gcc -mno-cygwin -I$(JAVA_HOME)/include -I$(JAVA_HOME)/include/win32 -g -Wl,--add-stdcall-alias -shared -o $@ $<