由于错误“thread 'main' paniced at 'called `Result::unwrap()` on an Err value，无法创建通用 JNIEnv

Question

我想创建一个带有固定值 JString（例如 "fake javastring"）的通用 JNIEnv（连同 JObject）来测试一些函数需要 JEnv 和 JObject 才能正常工作。我不想运行 Java 虚拟机。

我尝试创建通用 None-值 JNIEnv 但失败并显示错误：

failures:

---- main_tests::Jstrings::return_fake_string stdout ----
thread 'main_tests::Jstrings::return_fake_string' panicked at 'called `Result::unwrap()` on an `Err` value: Error(NullPtr("from_raw ptr argument"), State { next_error: None, backtrace: InternalBacktrace })', src/libcore/result.rs:999:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:39
   1: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at src/libstd/sys_common/backtrace.rs:59
             at src/libstd/panicking.rs:197
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:208
   4: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:474
   5: std::panicking::continue_panic_fmt
             at src/libstd/panicking.rs:381
   6: rust_begin_unwind
             at src/libstd/panicking.rs:308
   7: core::panicking::panic_fmt
             at src/libcore/panicking.rs:85
   8: core::result::unwrap_failed
             at /rustc/a53f9df32fbb0b5f4382caaad8f1a46f36ea887c/src/libcore/macros.rs:18
   9: core::result::Result<T,E>::unwrap
             at /rustc/a53f9df32fbb0b5f4382caaad8f1a46f36ea887c/src/libcore/result.rs:800
  10: rust::empty_jnienv
             at src/main.rs:10
  11: rust::main_tests::Jstrings::return_fake_string
             at src/main.rs:29
  12: rust::main_tests::Jstrings::return_fake_string::{{closure}}
             at src/main.rs:27
  13: core::ops::function::FnOnce::call_once
             at /rustc/a53f9df32fbb0b5f4382caaad8f1a46f36ea887c/src/libcore/ops/function.rs:231
  14: <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once
             at /rustc/a53f9df32fbb0b5f4382caaad8f1a46f36ea887c/src/liballoc/boxed.rs:704
  15: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:85
  16: test::run_test::run_test_inner::{{closure}}
             at /rustc/a53f9df32fbb0b5f4382caaad8f1a46f36ea887c/src/libstd/panicking.rs:272
             at /rustc/a53f9df32fbb0b5f4382caaad8f1a46f36ea887c/src/libstd/panic.rs:394
             at src/libtest/lib.rs:1468

我的代码 - Cargo.toml

[package]
name = "rust"
version = "0.1.0"
edition = "2018"

[dependencies]
libc = "0.2"
jni = { version = "0.10.2", default-features = false }

main.rs

mod backend;

extern crate libc;

use jni::objects::{JObject, JString};
use jni::JNIEnv;
use std::ptr;

pub unsafe fn empty_jnienv() -> jni::JNIEnv<'static> {
    jni::JNIEnv::from_raw(ptr::null_mut()).unwrap()
}

pub unsafe fn empty_jobj() -> jni::objects::JObject<'static> {
    jni::objects::JObject::null()
}

fn main() {}

#[cfg(test)]
mod main_tests {
    use super::*;

    mod Jstrings {
        use super::*;

        #[test]
        fn return_fake_string() {
            let this_string = "fake javastring";
            let jenv = unsafe { empty_jnienv() };
            let jobj = unsafe { empty_jobj() };
            let jstr: jni::objects::JString = jenv.new_string(this_string.to_owned()).unwrap();
            assert!(unsafe {
                backend::Java_com_example_android_MainActivity_test(jenv, jobj, jstr) == true
            });
        }
    }
}

backend.rs

use jni::objects::{JObject, JString};
use jni::JNIEnv;
use std::ffi::{CStr, CString};

#[no_mangle]
pub unsafe extern "C" fn Java_com_example_android_MainActivity_test(
    env: JNIEnv,
    _: JObject,
    j_recipient: JString,
) -> bool {
    let mut this_return = false;
    let recipient = CString::from(CStr::from_ptr(
        env.get_string(j_recipient).unwrap().as_ptr(),
    ));

    if recipient.to_str().unwrap() == "fake javastring" {
        this_return = true;
    }
    this_return
}

有没有办法在不运行宁 Java 虚拟机创建 JNIEnv 和 JObject 的情况下解决这个问题？

Answer 1

正如JNIEnv::from_raw()的文档所说，它并没有做太多，但是

Create a JNIEnv from a raw pointer.

Only does a null check - otherwise assumes that the pointer is valid.

所以传递 ptr::null_mut() 已经触发了这个，然后你的错误消息的第一行告诉同样的：

called `Result::unwrap()` on an `Err` value: Error(NullPtr("from_raw ptr argument") [...]

字面意思是空指针导致错误，而不是JNIEnv，因此不能unwrap()。

TL;DR：不，你所做的是行不通的，Java 没有 Java 是行不通的，并且基于代码的某些后面部分，可能值得添加 [=如果没有 Android，27=] 也将无法工作。即使您正确配置和使用 Java，拥有它运行和 Activity 将是一个全新的挑战。

Answer 2

如果你查看 jni.h，你会看到 JNIEnv* 是一个充满函数指针的大结构：

struct JNINativeInterface_ {
    void *reserved0;
    void *reserved1;
    void *reserved2;

    void *reserved3;
    jint (JNICALL *GetVersion)(JNIEnv *env);

    jclass (JNICALL *DefineClass)
      (JNIEnv *env, const char *name, jobject loader, const jbyte *buf,
       jsize len);
    jclass (JNICALL *FindClass)
      (JNIEnv *env, const char *name);

    // and so on
};

typedef const struct JNINativeInterface_ *JNIEnv;

如果你真的想走这条路，你可以只实现你的代码需要的功能，并将它们分配给 JNINativeInterface_ 的各个成员。我不太了解您使用的 Rust 桥，但是转换这样的 JNIEnv 指针应该可以。

当然，最后一个问题是：你实际测试的是什么？

由于错误“thread 'main' paniced at 'called `Result::unwrap()` on an Err value，无法创建通用 JNIEnv

Can't create a generic JNIEnv due to the error "thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value

java-native-interface

rust