Integrating Java and Native Programs

Mapping between Java and Native Types

In this section, you will learn how to reference Java types in your native method. You need to do this when you want to:

• Access arguments passed in to a native method from a Java application.
• Create Java objects in your native method.
• Have your native method return results to the caller.

Java Primitive Types

Your native method can directly access Java primitive types such as booleans, integers, floats, and so on, that are passed from programs written in Java. For example, the Java type boolean maps to the native language type jboolean (represented as unsigned 8 bits), while the Java type float maps to the native language type jfloat (represented by 32 bits). The following table describes the mapping of Java primitive types to native types.

Primitive Types and Native Equivalents

Java Type	Native Type	Size in bits
boolean	jboolean	8, unsigned
byte	jbyte	8
char	jchar	16, unsigned
short	jshort	16
int	jint	32
long	jlong	64
float	jfloat	32
double	jdouble	64
void	void	n/a

Java Object Types

Java objects are passed by reference. All references to Java objects have the type jobject. For convenience and to avoid programming errors, the JNI implements a set of types that are conceptually all subclassed from (or are "subtypes" of) jobject, as follows:

In our Prompt.java example, the native method getLine takes a Java string as an argument and returns a Java string:

private native String getLine(String prompt);

Its corresponding native implementation has type jstring for both the argument and the return value:

JNIEXPORT jstring JNICALL Java_Prompt_getLine(JNIEnv *, jobject, jstring);

Graphically, this looks as follows:

As mentioned above, jstring corresponds to the Java type String. Notice that the second argument to Java_Prompt_getLine, which is the reference to the object itself, has type jobject.

Integrating Java and Native Programs