rust-jni-demo/native/mylib/src/lib.rs

extern crate jni;
// This is the interface to the JVM that we'll call the majority of our
// methods on.
use jni::JNIEnv;
// These objects are what you should use as arguments to your native
// function. They carry extra lifetime information to prevent them escaping
// this context and getting used after being GC'd.
use jni::objects::{JClass, JString};
// This is just a pointer. We'll be returning it from our function. We
// can't return one of the objects with lifetime information because the
// lifetime checker won't let us.
use jni::sys::{jstring, jint};

use std::any::Any;
// This keeps rust from "mangling" the name and making it unique for this
// crate.
#[no_mangle]
// This turns off linter warnings because the name doesn't conform to
// conventions.
#[allow(non_snake_case)]
pub extern "system" fn Java_HelloWorld_hello(env: JNIEnv,
// This is the class that owns our static method. It's not going to be used,
// but still must be present to match the expected signature of a static
// native method.
                                             class: JClass,
                                             input: JString)
                                             -> jstring {
    // First, we have to get the string out of java. Check out the `strings`
    // module for more info on how this works.
    let input: String =
        env.get_string(input).expect("Couldn't get java string!").into();
    // Then we have to create a new java string to return. Again, more info
    // in the `strings` module.
    let output = env.new_string(format!("Hello, {}!", input))
        .expect("Couldn't create java string!");
    // Finally, extract the raw pointer to return.
    output.into_inner()
}

#[no_mangle]
#[allow(non_snake_case)]
pub extern "system" fn Java_MyMath_add(env: JNIEnv, class: JClass, a: jint, b: jint) -> jint {
    return a + b;
}