Java Native Interface (JNI) Tutorial - Hell on Stage

Intro

There are so many tutorials about using JNI to access C/C++ code from Java so I don't want to write the next ... but to share some tips & tricks with a very short overview.

Java and C together! Why?

1. Implement time-critical code
2. Access legacy code or code libraries from Java programs
3. Need of platform-dependent features not supported in Java (not covered)

Calling C/C++ code from Java programs

Six steps to do this with Java Native Interface:

1. Write Java code
   public class Sample{
public native String stringMethod(String text);
public static void main(String[] args) {
System.loadLibrary("sample");
Sample sample = new Sample();
String text = sample.stringMethod("java");
System.out.println("stringMethod: " + text)
}

2. Compile the Java code
   javac Sample.java
   
3. Generate C/C++ header file
   Generate Sample.h from the java class file: $javah -jni Sample. The result uses a pointer to a table of function pointers (JNIEnv parameter) and a parameter refers to the current invoking Java object or this pointer (jobject) :
   #include
extern "C" {
JNIEXPORT jstring JNICALL Java_Sample_stringMethod
(JNIEnv *, jobject, jstring);
}
   
4. Write C/C++ code
   Implement the methods from the generated header files:
   JNIEXPORT jstring JNICALL Java_Sample_stringMethod(JNIEnv *env, jobject obj, jstring string) {
const char *str = env->GetStringUTFChars(string, 0);
char cap[128];
strcpy(cap, str);
env->ReleaseStringUTFChars(string, str);
return env->NewStringUTF(strupr(cap));
}
   
5. Create shared library file
   The naming convension for the output shared library is different depending on the OS and must be followed: sample.dll (Windows) or libsample.so (*NIX).
   $g++ -I /usr/lib/jvm/java-1.5.0-sun/include sample.cpp -L ~/resources/verimatrix -shared -o libsample.so -lcrypto
   
6. Run the Java application
   java Sample

Access shared libraries

Prerequisites
Usually you have some shared libraries that provide some functionality. For example two dynamic linux shared libraries (libvpr.so and libringdll.so).
Problems

1. Difference between the provided method names in the libraries and the naming convension used by JNI - you cannot invoke one of the methods directly unless it is designed with JNI in mind. JNI requires some prefixes as shown above with the method name "Java_Sample_stringMethod"
2. Not all JNI data types can be mapped to C/C++ data types and vice versa:
   jint == int, jbyte == jbyte
   jbyteArray != byte *, jstring != string, jclass != class
3. Freeing Native Resources - some resources cannot be freed by the GC like strings and global references.

Access C/C++ methods provided by the libraries
The solution that handles the above problems is to implement a proxy library that follows the JNI naming convention and proxies all method calls to the functional shared library:

1. Implement Java code with all required native methods:
   private native boolean VprEncryptMovie(EncryptMovieSettings encryptionSettings);
2. Design C/C++ shared library that implement the interface of the methods as proxy:
   jboolean Java_jni_VerimatrixClient_VprEncryptMovie(EncryptMovieSettings jobject);
3. Implement data conversion from the JNI data types to standard C/C++ data types
4. Proxy all method calls to the corresponding methods in functional libraries
5. Implement exception handling
   jclass newExc=env->FindClass("java/lang/IllegalArgumentException");
env->ThrowNew(newExc, "thrown from C code");

Tips and tricks

1. Use Apache Harmony VM for trouble shooting or the patched version as described here - it generates a JNI stack trace instead of just crashing the VM as JDK5 or JDM6 eg:
   
   SIGSEGV in VM code.
Stack trace:
0: memcpy (??:-1)
1: VprRpcEncryptMovieWithKey(rpc_handle_s_t*, void*, unsigned char*, long, long, long, long, long, long, long, long, long, long, long, long, long, long, long, long, long, unsigned char*, long, long, long*, long*) (??:-1)
2: VprEncryptMovieWithKey (??:-1)
3: Java_com_minerva_edc_vig_verimatrix_vcas_VprClient_VprEncryptMovieWithKey (/home/emo/perforce/3ML/SB/VideoIngest/src/com/minerva/edc/vig/verimatrix/proxy/VprProxy.cpp:100)
4: 0xA591A50B
5: com/minerva/edc/vig/verimatrix/vcas/VprClient.createEncryptSession(Lcom/minerva/edc/vig/verimatrix/vcas/model/VprServerRegisterResponse;Lcom/minerva/edc/vig/verimatrix/vcas/model/EncryptMovieSettings;Lcom/minerva/edc/vig/verimatrix/vcas/model/MovieInfo;Lcom/minerva/edc/vig/core/data/DTO;)J (VprClient.java:73)
...
23: _start (/usr/src/packages/BUILD/glibc-2.3/csu/../sysdeps/i386/elf/start.S:105)
3. Eclipse plugin for C/C++ development - CDT
4. Do not store local variables in the C++ classes if more that one simultaneous client to the proxy is planned - only one instance of this class from the library is loaded by the JVM.
5. You can implement a Java class to pass multiple parameters to a JNI function and extract them with reflection but remmember that every reflection call has big performance hit. Also if this input parameter is "converted" into a C/C++ structure or class, the constructed objects cannot be cached (see the previos tip). If the parametes are not so many - pass them all.
   
6. A Java class can be returned as out parameter by reference. It is better to create the instance in the Java and just fill the properties in the C++ proxy. This eliminates the need to call the constructor of the response object and to hardcode its package and class name in the proxy class. Only the method names are required.


7. A setter method can be invoked in such a way:
   
   // Create the encryption settings class
jclass vprResponseClass = env->GetObjectClass(vprResponseObj);
// Allocates memory for an array of one long long output parameter
jvalue* args = (jvalue*)malloc(sizeof(jlong)); way:
// Create the encryption settings class
jclass vprResponseClass = env->GetObjectClass(vprResponseObj);
// Allocates memory for an array of one long long output parameter
jvalue* args = (jvalue*)malloc(sizeof(jlong));

args[0].j=input_file_size;
jmethodID setInputFileSizeID =env->GetMethodID(vprResponseClass,"setInputFileSize", "(J)V");
env->CallLongMethodA(vprResponseObj, setInputFileSizeID, args);
// Cleanup
free((jvalue*) args);



8. A thrown exception in a C++ method do not stops its execution so all allocated resources in the method can be freed first before the return clause.


9. Passing Invalid Arguments to JNI Functions - most often JVM crashes. Use JVM option -Xcheck:jni to detect errors like passing passing NULL or (jobject)0xFFFFFFFF in place of reference. This option degrates the performance.

Troubleshooting

java.lang.UnsatisfiedLinkError: no sample in java.library.path
The library name in Java (System.loadLibrary)is wrong - chech the prefix or extension
Could not find the library - set the directory locations as a JVM parameter: -Djava.library.path=/home/lib:/opt/lib/

java.lang.UnsatisfiedLinkError: /libverimatrixproxy.so: libvpr.so: cannot open shared object file: No such file or directory
A library dependency cannot be found. Add the location of the missing library e.g. ~/lib/ in "/etc/ld.so.conf". Then reconfigure linker bindings $ldconfig

java.lang.UnsatisfiedLinkError: /libverimatrix.so: /libverimatrix.so: undefined symbol: VprDestroyContext
Check the shared library for all undefined symbols in the list of its dependences
$ldd -d ~/lib/libverimatrix.so
Place the missing library that provides those methods, structres, enumerations or ... in the libraries dependency search path.

java.lang.UnsatisfiedLinkError: /libverimatrix.so: Can't load IA 32-bit .so on a IA 32-bit platform
Bug in JDK5 - wrong error message. Use another VM (JDK6, Harmony) to get the correct stack trace.

[Too many errors, abort]
Infinite error message dump:
Many times: [error occurred during error reporting, step 270, id 0xb]
Then infinite: [Too many errors, abort]
This occurs when the JDK detects an error in called C++ code but does not crash imediate. See the first tip about the Apache Harmony JVM.

error: redefinition of ‘struct EncryptMovieSettings’
This type of C++ compile error may happpen if a header file is called or included more than once. Try to precede and end it like that to avoid that:
#ifndef __yourheader_h
#define __yourheader_h (1)
// Put here the body of your ".h" file including the class
#endif

Debug JNI

Integrated Debugger for JNI Environments
Eclipse version 3.2 (not a newer) can be used to debug both the Java and C++ code of JNI. Apache Harmony JVM provides interface for agent which could manage and handle events in debug session.
Synchronized Sollution
Solution with two debuggers that attach to a running Java or C++ process and cooperate -  a Java debugger eg. Eclipse and a C/C++ debugger eg. Insight - GUI frondend to GDB

References

Java Native Interface 1999.pdf
Java programming with JNI.pdf

GMAIL over IMAP! The battle is over!

     Finally GMAIL will have IMAP support! 
I'm so tired of this battle with the POP3 access. And the best is that all win :)

     I started using GMAIL some years ago but lately the need to check all my three accounts through the web interface with the enormous loading time almost made me give up these mails.
I tried the POP forwarding to another mail account with IMAP support but could not find such proper mail service. 

     The winner was AOL mail  with unlimited space but after some doubt I found that the mails are kept for 28 days only before complete deletion. At second place was www.bluebottle.com with 250MB space but I couldn't register receiving some error with wrong message:There have been too many signups from your IP address in the last 24 hours. All other mail servers offered for free only no more than 20-30MB of space ~= 5-6 mails with attachments and I returned to the old painful POP3 access.

Tools for every day from JDK 6

JDK 6 introduced some usefull tools:

1. JPS - (Also incuded in JDK 5) lists all running JVMs (also those loaded from JNI !!!) with information for them as passed arguments and the  name of the application's main class with its arguments:
$jps -mlvV
3370 sun.tools.jps.Jps -mlvV -Dapplication.home=/usr/lib/jvm/java-6-sun-1.6.0.00 -Xms8m
30707 -Xms512m -Xmx512m -XX:PermSize=128m -XX:MaxPermSize=128m
3327 tcpmon.MainWindow

2. JINFO - prints Java configuration information for a given Java process. The most usefull info as sun.boot.library.path, java.library.path and java.class.path can be filtered:
$jinfo 30707| grep -E "(java.class.path|library.path)"

Those tools can be combined to expract the most usefull information for all JDK6 in a BASH script ( jpall.sh)  skipping the JPS process itself :
#!/bin/bash
for i in $( jps | grep -v Jps | awk -F " " '{print $1}' ) ; 
do
   echo
   jinfo $i | grep -E "(java.class.path|library.path)"
   jps -mlvV | grep $i
done