How do I get a Java field using the Rust jni crate?

Question

I am using the Rust jni crate, and currently have a native method defined. I am trying to figure out how I get a java field, which is a different class that I have also made, and interact with it inside my Rust code.

I have tried the following based on the small amount of documentation I could find for this crate and the JNI in general:

#[no_mangle]
pub extern "system" fn Java_name__11nteresting_example_FirstClass_example<'local>(mut env: JNIEnv<'local>, class: JClass<'local>) {
    let field_id: jni::objects::JFieldID = env.get_field_id(class, "fieldName", "Lname/_1nteresting/example/SecondClass;").expect("Unable to get field ID");
    let field = env.get_field(class, "fieldName", field_id);
}

Currently, I am getting the error:

the trait bound `JFieldID: AsRef<str>` is not satisfied
the trait `AsRef<str>` is not implemented for `JFieldID`
but trait `AsRef<JFieldID>` is implemented for it
for that trait implementation, expected `JFieldID`, found `str`

on the field_id when I give it to the get_field function. I assume that I probably have misread what I could find about this, although the jni crate doesn't seem to have much documentation about this.

Botje · Accepted Answer · 2025-04-18 20:07:56Z

0

From your code it looks like you're trying to access class field (static in Java), in which case you want to use get_static_field instead. The documentation for get_static_field states that it does an class lookup and a field id lookup internally.

So you can write your code as:

env.get_static_field(class, "fieldName", "Lname/_1nteresting/example/SecondClass;");

If you wanted to access an instance field, you need to have an object in hand and use get_field instead.

answered Apr 18 at 20:07

Botje

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