refactor(java): simpilfy fragment (#3307)

In PR #3240, python code is
refactored, fragment is dataclass now. This PR refactors Java code, make
the API consistent with python api.

java/core/lance-jni/src/blocking_dataset.rs

@@ -14,7 +14,7 @@
 
 use crate::error::{Error, Result};
 use crate::ffi::JNIEnvExt;
-use crate::traits::FromJString;
+use crate::traits::{export_vec, import_vec, FromJObjectWithEnv, FromJString};
 use crate::utils::{extract_storage_options, extract_write_params, get_index_params};
 use crate::{traits::IntoJava, RT};
 use arrow::array::RecordBatchReader;
@@ -355,7 +355,7 @@ pub extern "system" fn Java_com_lancedb_lance_Dataset_commitAppend<'local>(
     _obj: JObject,
     path: JString,
     read_version_obj: JObject,    // Optional<Long>
-    fragments_obj: JObject,       // List<String>, String is json serialized Fragment
+    fragments_obj: JObject,       // List<FragmentMetadata>
     storage_options_obj: JObject, // Map<String, String>
 ) -> JObject<'local> {
     ok_or_throw!(
@@ -374,31 +374,18 @@ pub fn inner_commit_append<'local>(
     env: &mut JNIEnv<'local>,
     path: JString,
     read_version_obj: JObject,    // Optional<Long>
-    fragments_obj: JObject,       // List<String>, String is json serialized Fragment)
+    fragment_objs: JObject,       // List<FragmentMetadata>
     storage_options_obj: JObject, // Map<String, String>
 ) -> Result<JObject<'local>> {
-    let json_fragments = env.get_strings(&fragments_obj)?;
-    let mut fragments: Vec<Fragment> = Vec::new();
-    for json_fragment in json_fragments {
-        let fragment = Fragment::from_json(&json_fragment)?;
-        fragments.push(fragment);
+    let fragment_objs = import_vec(env, &fragment_objs)?;
+    let mut fragments = Vec::with_capacity(fragment_objs.len());
+    for f in fragment_objs {
+        fragments.push(f.extract_object(env)?);
     }
     let op = Operation::Append { fragments };
     let path_str = path.extract(env)?;
     let read_version = env.get_u64_opt(&read_version_obj)?;
-    let jmap = JMap::from_env(env, &storage_options_obj)?;
-    let storage_options: HashMap<String, String> = env.with_local_frame(16, |env| {
-        let mut map = HashMap::new();
-        let mut iter = jmap.iter(env)?;
-        while let Some((key, value)) = iter.next(env)? {
-            let key_jstring = JString::from(key);
-            let value_jstring = JString::from(value);
-            let key_string: String = env.get_string(&key_jstring)?.into();
-            let value_string: String = env.get_string(&value_jstring)?.into();
-            map.insert(key_string, value_string);
-        }
-        Ok::<_, Error>(map)
-    })?;
+    let storage_options = extract_storage_options(env, &storage_options_obj)?;
     let dataset = BlockingDataset::commit(&path_str, op, read_version, storage_options)?;
     dataset.into_java(env)
 }
@@ -410,7 +397,7 @@ pub extern "system" fn Java_com_lancedb_lance_Dataset_commitOverwrite<'local>(
     path: JString,
     arrow_schema_addr: jlong,
     read_version_obj: JObject,    // Optional<Long>
-    fragments_obj: JObject,       // List<String>, String is json serialized Fragment
+    fragments_obj: JObject,       // List<FragmentMetadata>
     storage_options_obj: JObject, // Map<String, String>
 ) -> JObject<'local> {
     ok_or_throw!(
@@ -431,14 +418,13 @@ pub fn inner_commit_overwrite<'local>(
     path: JString,
     arrow_schema_addr: jlong,
     read_version_obj: JObject,    // Optional<Long>
-    fragments_obj: JObject,       // List<String>, String is json serialized Fragment)
+    fragments_obj: JObject,       // List<FragmentMetadata>
     storage_options_obj: JObject, // Map<String, String>
 ) -> Result<JObject<'local>> {
-    let json_fragments = env.get_strings(&fragments_obj)?;
-    let mut fragments: Vec<Fragment> = Vec::new();
-    for json_fragment in json_fragments {
-        let fragment = Fragment::from_json(&json_fragment)?;
-        fragments.push(fragment);
+    let fragment_objs = import_vec(env, &fragments_obj)?;
+    let mut fragments = Vec::with_capacity(fragment_objs.len());
+    for f in fragment_objs {
+        fragments.push(f.extract_object(env)?);
     }
     let c_schema_ptr = arrow_schema_addr as *mut FFI_ArrowSchema;
     let c_schema = unsafe { FFI_ArrowSchema::from_raw(c_schema_ptr) };
@@ -596,14 +582,14 @@ fn inner_open_native<'local>(
 }
 
 #[no_mangle]
-pub extern "system" fn Java_com_lancedb_lance_Dataset_getJsonFragments<'a>(
+pub extern "system" fn Java_com_lancedb_lance_Dataset_getFragmentsNative<'a>(
     mut env: JNIEnv<'a>,
     jdataset: JObject,
 ) -> JObject<'a> {
-    ok_or_throw!(env, inner_get_json_fragments(&mut env, jdataset))
+    ok_or_throw!(env, inner_get_fragments(&mut env, jdataset))
 }
 
-fn inner_get_json_fragments<'local>(
+fn inner_get_fragments<'local>(
     env: &mut JNIEnv<'local>,
     jdataset: JObject,
 ) -> Result<JObject<'local>> {
@@ -612,22 +598,37 @@ fn inner_get_json_fragments<'local>(
             unsafe { env.get_rust_field::<_, _, BlockingDataset>(jdataset, NATIVE_DATASET) }?;
         dataset.inner.get_fragments()
     };
+    let fragments = fragments
+        .iter()
+        .map(|f| f.metadata().clone())
+        .collect::<Vec<Fragment>>();
+    export_vec(env, &fragments)
+}
 
-    let array_list_class = env.find_class("java/util/ArrayList")?;
-
-    let array_list = env.new_object(array_list_class, "()V", &[])?;
+#[no_mangle]
+pub extern "system" fn Java_com_lancedb_lance_Dataset_getFragmentNative<'a>(
+    mut env: JNIEnv<'a>,
+    jdataset: JObject,
+    fragment_id: jint,
+) -> JObject<'a> {
+    ok_or_throw!(env, inner_get_fragment(&mut env, jdataset, fragment_id))
+}
 
-    for fragment in fragments {
-        let json_string = serde_json::to_string(fragment.metadata())?;
-        let jstring = env.new_string(json_string)?;
-        env.call_method(
-            &array_list,
-            "add",
-            "(Ljava/lang/Object;)Z",
-            &[(&jstring).into()],
-        )?;
-    }
-    Ok(array_list)
+fn inner_get_fragment<'local>(
+    env: &mut JNIEnv<'local>,
+    jdataset: JObject,
+    fragment_id: jint,
+) -> Result<JObject<'local>> {
+    let fragment = {
+        let dataset =
+            unsafe { env.get_rust_field::<_, _, BlockingDataset>(jdataset, NATIVE_DATASET) }?;
+        dataset.inner.get_fragment(fragment_id as usize)
+    };
+    let obj = match fragment {
+        Some(f) => f.metadata().into_java(env)?,
+        None => JObject::default(),
+    };
+    Ok(obj)
 }
 
 #[no_mangle]