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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.
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 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:
Its corresponding native implementation has typeprivate native String getLine(String prompt);
jstring
for both the argument and the return value:
JNIEXPORT jstring JNICALL Java_Prompt_getLine(JNIEnv *, jobject, jstring);
Graphically, this looks as follows:
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
.
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