When running in JDK6 environment, FindClass failed for java/util/OptionalDouble error occurs

[freelife1191]

I am using it in a JDK6 environment and I would like to know how to remove java.util.Optional related classes during build.

Is there a way to disable java.util.Optional for running in a JDK6 environment?

I tried to remove only java.util.OptionalDouble by adding this code to jni_typemaps.rs and building, but java.util.OptionalDouble is not removed.

jni_typemaps.rs

mod swig_foreign_types_map {}

#[allow(dead_code)]
fn to_java_util_opt_double(env: *mut JNIEnv, x: Option<f64>) -> f64 {
    match x {
        Some(val) => val,
        None => 0.0,
    }
}

foreign_typemap!(
    ($p:r_type) Option<f64> => f64 {
        $out = to_java_util_opt_double(env, $p);
    };
    (f_type, option = "NoNullAnnotations") => "double";
    (f_type, option = "NullAnnotations") => "@NonNull double";
);

foreign_typemap!(
    ($p:r_type) Option<f32> => f32 {
        $out = to_java_util_opt_double(env, $p.map(f64::from));
    };
);

Below is the main function code of build.rs

fn main() {
    env_logger::init();

    let out_dir = env::var("OUT_DIR").unwrap();
    let jni_c_headers_rs = Path::new(&out_dir).join("jni_c_header.rs");
    gen_jni_bindings(&jni_c_headers_rs);

    let have_java_9 = fs::read_to_string(&jni_c_headers_rs)
        .unwrap()
        .contains("JNI_VERSION_9");

    let java_cfg = JavaConfig::new(
        Path::new("lib")
            .join("src")
            .join("main")
            .join("java")
            .join("com")
            .join("example"),
        "com.example".into())
    //.use_optional_package("com.google.common.base".into())
    .use_reachability_fence(if have_java_9 {
        JavaReachabilityFence::Std
    } else {
        JavaReachabilityFence::GenerateFence(8)
    });

    let out_dir = env::var("OUT_DIR").unwrap();
    let in_src = Path::new("src").join("java_glue.rs.in");
    let out_src = Path::new(&out_dir).join("java_glue.rs");
    let flap_gen = flapigen::Generator::new(LanguageConfig::JavaConfig(java_cfg))
        .merge_type_map("typemaps", include_str!("src/jni_typemaps.rs"));
    flap_gen.expand("java bindings", &in_src, &out_src);
}

Errors that occur when running in a JDK6 environment

thread '<unnamed>' panicked at /target/x86_64-unknown-linux-gnu/release/build/enigma-0323bbf5eaf7b330/out/java_glue.rs:11:3134:
FindClass failed for java/util/OptionalDouble
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread '<unnamed>' panicked at library/core/src/panicking.rs:221:5:
panic in a function that cannot unwind
stack backtrace:
   0:     0x735c9dc805d5 - <unknown>
   1:     0x735c9dcabdcb - <unknown>
   2:     0x735c9dc7d31f - <unknown>
   3:     0x735c9dc818d1 - <unknown>
   4:     0x735c9dc815ac - <unknown>
   5:     0x735c9dc81fa1 - <unknown>
   6:     0x735c9dc81dd3 - <unknown>
   7:     0x735c9dc80a99 - <unknown>
   8:     0x735c9dc81a94 - <unknown>
   9:     0x735c9dca9795 - <unknown>
  10:     0x735c9dca9822 - <unknown>
  11:     0x735c9dca9966 - <unknown>
  12:     0x735c9d725f9b - JNI_OnLoad
  13:     0x735cacb8ad0c - Java_java_lang_ClassLoader_00024NativeLibrary_load
thread caused non-unwinding panic. aborting.
Aborted (core dumped)

Could you please tell me the build configuration to remove java.util.OptionalX so that it can work properly on JDK6?