JNIEnv
- Java course
- Expert
- Lesson 27/46
- 5 min
Learn the native call environment with JNIEnv.
The JNIEnv interface
The first parameter of every JNI C function is JNIEnv*. This pointer lets C talk to the JVM: calling Java methods, creating objects and converting arrays and strings all go through env->....
For static native methods the second parameter is jclass (the class); for instance methods it is jobject (this). Without it C would not know which class or object it works on.
C function naming
To match a C function with a Java method JNI uses a fixed name: Java_packageName_ClassName_methodName. Dots in the package become underscores. For example, NativeOps.sum maps to the C function Java_NativeOps_sum.
For overloaded methods a signature suffix (such as __II) is appended to avoid name clashes. This naming is the backbone of the JNI linking mechanism.
Practice Write the class that builds the C function name
Define class JniCall with String className and String method fields and a JniCall(String className, String method) constructor that assigns them. A String cName() method returns "Java_" + className + "_" + method. In main create JniCall c = new JniCall("NativeOps", "sum"); and print it with System.out.println(c.cName());.
Starter code
// Define the JniCall class: className, method fields, constructor and cName method
// Main: create a JniCall instance and print the cName outputSample solution
class JniCall {
String className;
String method;
JniCall(String className, String method) {
this.className = className;
this.method = method;
}
String cName() {
return "Java_" + className + "_" + method;
}
}
public class Main {
public static void main(String[] args) {
JniCall c = new JniCall("NativeOps", "sum");
System.out.println(c.cName());
}
}Expected output
Java_NativeOps_sum
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match the JNI parameter types
| Term | Meaning |
|---|---|
JNIEnv* |
First parameter |
jclass |
Class in static method |
jobject |
this in instance method |
jint |
int counterpart |
2 Complete the C function name
Java_____(JNIEnv* env, jclass cls, jint a, jint b)Java_NativeOps_sum(JNIEnv* env, jclass cls, jint a, jint b)Explanation: The C function is named Java_NativeOps_sum.
3 What is JNIEnv* for?
- It compiles the class
- It starts threads
- It reads files
- It lets C reach the JVM Correct answer
Explanation: Through JNIEnv* C calls Java methods and creates objects.