JNI Exercises

Simple example on macOS

Create the java file. HelloWorld.java

Execute javac HelloWorld.java , this generate a HelloWorld.class file.

After that, generate the C Header executing javah -jni HelloWorld, you'll get HelloWorld.h file.

Create a C file. HelloWorld.c.

Implement the C functions.

Compile C file with this command on macOS.

gcc -c -Wall -Werror -fpic HelloWorld.c -I${JAVA_HOME}/include -I${JAVA_HOME}/include/darwin

-I flag adds other C files needed like jni.h and jni_md.h.

This command will generate a HelloWorld.o file.

Then, you should create a shared library from the *.o file using this command.

gcc -shared -o libHelloWorld.dylib HelloWorld.o

This command will generate a libHelloWorld.dylib file.

If you have all the files on the same directory you should have the following files.

• HelloWorld.java
• HelloWorld.class
• HelloWorld.h
• HelloWorld.c
• HelloWorld.o
• HelloWorld.dylib

Finally, Execute the java class.

java HelloWorld -Djava.library.path=libHelloWorld.dylib

Recipes

Recipe 01 HelloWorld

The first recipe is a simple HelloWorld.

Recipe 02 Native Types

In this recipe I'm passing the Java Primitive Types to C where will be converted to the appropiate Native Type. More information

Java Primitive Types	JNI Types	C Type	Description
byte	jbyte	signed char	signed 8 bits
short	jshort	short	signed 16 bits
int	jint	int	signed 32 bits
long	jlong	long long	signed 64 bits
float	jfloat	float	32 bits
double	jdouble	double	64 bits
boolean	jboolean	unsigned char	unsigned 8 bits
char	jchar	unsigned short	unsigned 16 bits
void	void	N/A	N/A

signed means that the type can represent negavite numbers; unsigned cannot.

Recipe 03 Strings

In this recipe I'm passing Strings and using some JNI methods like:

UTF-8

• GetStringUTFChars() Convert jstring to const char *.
• ReleaseStringUTFChars() Release memory
• NewStringUTF() Create new string
• GetStringUTFLength() get the jsize of the string.

Unicode

• GetStringChars() Convert jstring to const char *.
• ReleaseStringChars() Release memory
• NewString() Create new string
• GetStringLength() get the jsize of the string.

jsize is an alias, its real type is jint.

typedef jint jsize;

More information

Recipe 04 Arrays

• GetArrayLength(env, array)

  Returns the number of elements in the array.

  jsize length = (*env)->GetArrayLength(env, array);

• Get[ PrimitiveType ]ArrayElements(env, array, 0) Routines

  jboolean *body = (*env)->GetBooleanArrayElements(env, array, 0);

• New[ PrimitiveType ]Array Routines

  A family of operations used to construct a new primitive array object.

  NewArray Routines	Array Type
  NewBooleanArray()	jbooleanArray
  NewByteArray()	jbyteArray
  NewCharArray()	jcharArray
  NewShortArray()	jshortArray
  NewIntArray()	jintArray
  NewLongArray()	jlongArray
  NewFloatArray()	jfloatArray
  NewDoubleArray()	jdoubleArray

  jintArray = (*env)->NewIntArray(env, size);

• Set[ PrimitiveType ]ArrayRegion Routines

  A family of functions that copies back a region of a primitive array from a buffer.

   jintArray  = (*env)->NewIntArray(env, size);
   
   jint start = 0;
   jint length = 10;
   jint tempArray [size];
   
   (*env)->SetIntArrayRegion(env, javaArray, start, length, tempArray);
  

• Release[ PrimitiveType ]ArrayElements(env, array, body, mode) Routines

mode	actions
0	copy back the content and free the elems buffer
JNI_COMMIT	copy back the content but dont free the elems buffer.
JNI_ABORT	free the buffer without copying back the possible changes

Recipe 05 Fields and Methods

Fields

Java supports two kinds of fields: instance fields and static fields.

• GetObjectClass(env, obj) Gets the class reference.

• GetFieldID(env, classReference, fieldName, fieldSignature) Gets the field ID.

• GetObjectField() Gets the field reference.

• SetObjectField() Sets the value of an instance (nonstatic) field of an object.

• GetStaticFieldID() Returns the field ID for a static field of a class.

• GetStatic[ PrimitiveType ]Field() Returns the value of a static field of an object.

• SetStatic[ PrimitiveType ]Field() Sets the value of a static field of an object.

JNI field signature

We can use the javap tool to geneate the field descriptors from the class files. Normally javap prints out the method and field types in a given class.

If we specify the -s option It will print the field descriptors.

If we specify the -p option It will print the field descriptors of private members. More Info

javap -s -p MyClass

Methods

• GetMethodID(env, classReference, methodname, signature) performs a lookup for the method in the given class.

• Call[ PrimitiveType ]Method() invokes an instance method that has the return type [ PrimitiveType ] or Object.

• GetStaticMethodID() performs a lookup for the static method in the given class.

• CallStatic[ PrimitiveType ]Method() invokes an static method that has the return type [ PrimitiveType ] or Object.

Recipe 06 Local and Global references

The JNI supports three kinds of opaque references: local references, global references, and weak global references.

Local and global references have different lifetimes. Local references are automatically freed, whereas global and weak global references remain valid until they are freed by the programmer.

A local or global reference keeps the referenced object from being garbage collected. A weak global reference, on the other hand, allows the referenced object to be garbage collected.

Local References

A local reference is valid only within the dynamic context of the native method that creates it, and only within that one invocation of the native method.

All local references created during the execution of a native method will be freed once the native method returns.

Local references are also only valid in the thread that creates them.

Global References

You can use a global reference across multiple invocations of a native method. A global reference can be used across multiple threads and remains valid until it is freed by the programmer. Like a local reference, a global reference ensures that the referenced object will not be garbage collected.

• NewGlobalRef(env, obj) is the only method to create global references.

• DeleteGlobalRef(env, obj) deletes a global reference.

Weak Global References

Like global refer- ences, weak global references remain valid across native method calls and across different threads. Unlike global references, weak global references do not keep the underlying object from being garbage collected.

• NewGlobalWeakRef(env, obj)

• DeleteGlobalWeakRef()

Recipe 07 Exceptions

• ExceptionCheck(env) Checks for pending exceptions without creating a local reference to the exception object. It returns JNI_TRUE or JNI_FALSE.

• ExceptionOccurred() Determines if an exception is being thrown. The exception stays being thrown until either the native code calls ExceptionClear(), or the Java code handles the exception.

  Returns the exception object that is currently in the process of being thrown, or NULL if no exception is currently being thrown.

• ExceptionDescribe() Prints an exception and a backtrace of the stack to a system error-reporting channel, such as stderr. This is a convenience routine provided for debugging.

• ExceptionClear() Clears any exception that is currently being thrown. If no exception is currently being thrown, this routine has no effect.

• ThowNew(env, exceptionClassRef, "Message")

More of JNI

JNIEnv interface pointer

Points to a location that contains a pointer to a function table. Each entry in the function table points to a JNI function. Native methods always access data structures in the Java virtual machine through one of the JNI functions.

jni.h and jni_md

jni.h contains the JNI types and functions to communicate Java and Native Libraries.

jni_md.h contains the machine-dependent typedefs for jbyte, jint and jlong.

C stuffs

typedef is a keyword in the C and C++ languages to create an alias name for another data type.

#define allows constant values.

#ifdef and #endif directive allows for conditional compilation

References

http://journals.ecs.soton.ac.uk/java/tutorial/native1.1/implementing/array.html

https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html