Add FFI-compatible Return<'env, T> for returning objects from a nat…

…ive method call.

java-spaghetti/src/lib.rs

@@ -12,16 +12,17 @@ pub use ::jni_sys as sys;
 pub use ::std;
 
 mod refs {
-
     mod arg;
     mod global;
     mod local;
     mod ref_;
+    mod return_;
 
     pub use arg::*;
     pub use global::*;
     pub use local::*;
     pub use ref_::*;
+    pub use return_::*;
 }
 
 mod array;

java-spaghetti/src/refs/local.rs

@@ -3,7 +3,7 @@ use std::ops::Deref;
 
 use jni_sys::*;
 
-use crate::{Env, Global, Ref, ReferenceType};
+use crate::{Env, Global, Ref, ReferenceType, Return};
 
 /// A [Local](https://www.ibm.com/support/knowledgecenter/en/SSYKE2_8.0.0/com.ibm.java.vm.80.doc/docs/jni_refs.html),
 /// non-null, reference to a Java object (+ [Env]) limited to the current thread/stack.
@@ -70,6 +70,16 @@ impl<'env, T: ReferenceType> Local<'env, T> {
         self.ref_
     }
 
+    pub fn as_return(self) -> Return<'env, T> {
+        let env: *mut *const JNINativeInterface_ = self.env().as_raw();
+        let object = unsafe { ((**env).v1_2.NewLocalRef)(env, self.as_raw()) };
+        unsafe { Return::from_raw(object) }
+    }
+
+    pub fn into_return(self) -> Return<'env, T> {
+        unsafe { Return::from_raw(self.into_raw()) }
+    }
+
     pub fn cast<U: ReferenceType>(&self) -> Result<Local<'env, U>, crate::CastError> {
         let env = self.env();
         let jnienv = env.as_raw();

java-spaghetti/src/refs/return_.rs

@@ -0,0 +1,37 @@
+use std::marker::PhantomData;
+use std::ptr::null_mut;
+
+use jni_sys::jobject;
+
+use crate::ReferenceType;
+
+/// FFI: Use **Return\<java::lang::Object\>** instead of jobject.  This represents a (null?) JNI function call return value.
+///
+/// Unlike most Java reference types from this library, this *can* be null.
+///
+/// FFI safe where a jobject is safe, assuming you match your types correctly.
+#[repr(transparent)]
+pub struct Return<'env, T: ReferenceType> {
+    object: jobject,
+    _class: PhantomData<&'env T>,
+}
+
+impl<'env, T: ReferenceType> Return<'env, T> {
+    pub unsafe fn from_raw(object: jobject) -> Self {
+        Self {
+            object,
+            _class: PhantomData,
+        }
+    }
+
+    pub fn null() -> Self {
+        Self {
+            object: null_mut(),
+            _class: PhantomData,
+        }
+    }
+
+    pub fn as_raw(&self) -> jobject {
+        self.object
+    }
+}