JNIEnv

JNIEnv is a pointer to a structure of function pointers that expose the Java Native Interface (JNI) for a thread calling into native code. In C and C++ native methods, the first parameter is typically JNIEnv* env, through which all JNI operations are performed. The env pointer represents the per-thread JNI environment and is valid only for the duration of the thread’s attachment to the Java Virtual Machine (VMI) or the duration of the native method call.

A native thread must attach to the JVM to obtain a JNIEnv*. If the native code is invoked directly from Java, the JVM supplies the JNIEnv* for the duration of that call. If the native thread is created by native code, it must call AttachCurrentThread to obtain a JNIEnv*, and DetachCurrentThread when finished. The env pointer should not be cached for use across threads or across different attachments.

JNIEnv provides access to functions for interacting with Java objects and classes, including FindClass, GetMethodID, GetFieldID, NewObject, CallObjectMethod, and conversions between Java and native types (for example, GetStringUTFChars, ReleaseStringUTFChars). It also includes error and exception handling facilities, such as ExceptionOccurred, ExceptionDescribe, and ThrowNew. The environment also enables creation and manipulation of Java arrays and strings, and, via GetJavaVM, access to the underlying JavaVM for operations such as attaching threads or obtaining a new JNIEnv for other threads.

In summary, JNIEnv is the thread-specific interface to JNI, enabling native code to interact with Java code while accounting for the thread lifecycle and JVM attachment requirements.