Java Native Interface Programming

Mapping between Java and Native Types

In this section, you will learn how to reference Java types in your native method. This is useful when you want to access the arguments passed in from Java, create new Java objects, and 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 Java programs. For example, the Java type boolean maps to the native type jboolean (represented as unsigned 8 bits), while the Java type float maps to the native type jfloat (represented by 32 bits). The following table describes the mapping of Java primitive types to native types.

Java 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 type jobject. For convenience and to reduce the chance of programming errors, we introduce a set of types that are conceptually all "subtypes" of jobject:

• jobject represents all Java objects.
• jclass represents Java class objects (java.lang.Class).
• jstring represents Java strings (java.lang.String).
• jarray represents Java arrays.
• jobjectArray represents arrays of objects.
• jbooleanArray represents boolean arrays.
• jbyteArray represents byte arrays.
• jcharArray represents char arrays.
• jshortArray represents short arrays.
• jintArray represents int arrays.
• jlongArray represents long arrays.
• jfloatArray represents float arrays.
• jdoubleArray represents double arrays.
• jthrowable represents Java exceptions (java.lang.Throwable).

In our Prompt.java example, the native method getLine:

private native String getLine(String prompt);

takes a Java string as an argument and returns a Java string. Its corresponding native implementation has type:

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

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

Java Native Interface Programming