Java Native Interface (JNI)

The Java Native Interface (JNI) is the standard mechanism that allows Java code running in the JVM to call, and be called by, native code written in C, C++, or other languages that can produce a shared library. It has been part of Java since version 1.1.

JNI is used when Java needs to interact with platform-specific functionality (hardware access, OS APIs, existing C libraries), for performance-critical code where a native implementation is significantly faster, or to integrate with existing native codebases that cannot be rewritten in Java.

Using JNI requires writing glue code in C/C++ that bridges Java method signatures (represented as mangled function names with JNIEnv* and jobject parameters) to the native implementation. It is verbose, error-prone, and bypasses the JVM's memory safety guarantees — incorrect JNI code can crash the JVM or cause memory corruption.

For new code in Java 22+, the Foreign Function & Memory (FFM) API is the recommended alternative. FFM achieves the same goals — calling native functions, reading and writing native memory — entirely from Java, without any native glue code, and with explicit lifetime management that prevents many classes of memory safety bugs. JNI remains available for backwards compatibility.

See also: Foreign Function & Memory API, Bytecode