--- a
+++ b/tests/data/test_json.py
@@ -0,0 +1,291 @@
+import json
+from itertools import islice
+from pathlib import Path
+
+import pytest
+from typing_extensions import Literal
+
+import edsnlp
+
+
+def assert_doc_read(doc):
+    assert doc._.note_id == "subfolder/doc-1"
+    assert doc._.context_var == "test"
+
+    attrs = ("etat", "assertion")
+    spans_and_attributes = {
+        "__ents__": sorted(
+            [
+                (e.start, e.end, e.text, tuple(getattr(e._, key) for key in attrs))
+                for e in doc.ents
+            ]
+        ),
+        **{
+            name: sorted(
+                [
+                    (e.start, e.end, e.text, tuple(getattr(e._, key) for key in attrs))
+                    for e in doc.spans[name]
+                ]
+            )
+            for name in doc.spans
+        },
+    }
+
+    assert spans_and_attributes == {
+        "__ents__": [
+            (6, 7, "douleurs", (None, None)),
+            (7, 11, "dans le bras droit", (None, None)),
+            (17, 21, "problème \nde locomotion", (None, "absent")),
+            (25, 26, "AVC", ("passé", "non-associé")),
+            (35, 36, "rhume", ("présent", "hypothétique")),
+            (45, 46, "rhume", ("présent", "hypothétique")),
+            (51, 52, "Douleurs", (None, None)),
+            (52, 56, "dans le bras droit", (None, None)),
+            (68, 69, "anomalie", (None, "absent")),
+        ],
+        "anatomie": [
+            (9, 11, "bras droit", (None, None)),
+            (54, 56, "bras droit", (None, None)),
+        ],
+        "localisation": [
+            (7, 11, "dans le bras droit", (None, None)),
+            (52, 56, "dans le bras droit", (None, None)),
+        ],
+        "pathologie": [
+            (17, 21, "problème \nde locomotion", (None, "absent")),
+            (25, 26, "AVC", ("passé", "non-associé")),
+            (35, 36, "rhume", ("présent", "hypothétique")),
+            (45, 46, "rhume", ("présent", "hypothétique")),
+        ],
+        "sosy": [
+            (6, 7, "douleurs", (None, None)),
+            (51, 52, "Douleurs", (None, None)),
+            (68, 69, "anomalie", (None, "absent")),
+        ],
+    }
+
+
+def assert_doc_write(exported_obj):
+    assert exported_obj == {
+        "entities": [
+            {
+                "assertion": None,
+                "end_char": 38,
+                "etat": "test",
+                "lexical_variant": "douleurs",
+                "note_nlp_id": 0,
+                "note_nlp_source_value": "sosy",
+                "start_char": 30,
+            },
+            {
+                "assertion": None,
+                "end_char": 57,
+                "etat": None,
+                "lexical_variant": "dans le bras droit",
+                "note_nlp_id": 1,
+                "note_nlp_source_value": "localisation",
+                "start_char": 39,
+            },
+            {
+                "assertion": None,
+                "end_char": 57,
+                "etat": None,
+                "lexical_variant": "bras droit",
+                "note_nlp_id": 2,
+                "note_nlp_source_value": "anatomie",
+                "start_char": 47,
+            },
+            {
+                "assertion": "absent",
+                "end_char": 98,
+                "etat": None,
+                "lexical_variant": "problème \nde locomotion",
+                "note_nlp_id": 3,
+                "note_nlp_source_value": "pathologie",
+                "start_char": 75,
+            },
+            {
+                "assertion": "non-associé",
+                "end_char": 117,
+                "etat": "passé",
+                "lexical_variant": "AVC",
+                "note_nlp_id": 4,
+                "note_nlp_source_value": "pathologie",
+                "start_char": 114,
+            },
+            {
+                "assertion": "hypothétique",
+                "end_char": 164,
+                "etat": "présent",
+                "lexical_variant": "rhume",
+                "note_nlp_id": 5,
+                "note_nlp_source_value": "pathologie",
+                "start_char": 159,
+            },
+            {
+                "assertion": "hypothétique",
+                "end_char": 296,
+                "etat": "présent",
+                "lexical_variant": "rhume",
+                "note_nlp_id": 6,
+                "note_nlp_source_value": "pathologie",
+                "start_char": 291,
+            },
+            {
+                "assertion": None,
+                "end_char": 314,
+                "etat": None,
+                "lexical_variant": "Douleurs",
+                "note_nlp_id": 7,
+                "note_nlp_source_value": "sosy",
+                "start_char": 306,
+            },
+            {
+                "assertion": None,
+                "end_char": 333,
+                "etat": None,
+                "lexical_variant": "dans le bras droit",
+                "note_nlp_id": 8,
+                "note_nlp_source_value": "localisation",
+                "start_char": 315,
+            },
+            {
+                "assertion": None,
+                "end_char": 333,
+                "etat": None,
+                "lexical_variant": "bras droit",
+                "note_nlp_id": 9,
+                "note_nlp_source_value": "anatomie",
+                "start_char": 323,
+            },
+            {
+                "assertion": "absent",
+                "end_char": 386,
+                "etat": None,
+                "lexical_variant": "anomalie",
+                "note_nlp_id": 10,
+                "note_nlp_source_value": "sosy",
+                "start_char": 378,
+            },
+        ],
+        "note_id": "subfolder/doc-1",
+        "context_var": "test",
+        "note_text": "Le patient est admis pour des douleurs dans le bras droit, mais "
+        "n'a pas de problème \n"
+        "de locomotion. \n"
+        "Historique d'AVC dans la famille. pourrait être un cas de "
+        "rhume.\n"
+        "NBNbWbWbNbWbNBNbNbWbWbNBNbWbNbNbWbNBNbWbNbNBWbWbNbNbNBWbNbWbNbWBNbNbWbNbNBNbWb"
+        "WbNbWBNbNbWbNBNbWbWbNb\n"
+        "Pourrait être un cas de rhume.\n"
+        "Motif :\n"
+        "Douleurs dans le bras droit.\n"
+        "ANTÉCÉDENTS\n"
+        "Le patient est déjà venu\n"
+        "Pas d'anomalie détectée.\n",
+    }
+
+
+def test_read_in_worker(blank_nlp, tmpdir):
+    input_dir = Path(__file__).parent.parent.resolve() / "resources" / "docs.jsonl"
+    list(
+        edsnlp.data.read_json(
+            input_dir,
+            converter="omop",
+            span_attributes=["etat", "assertion"],
+            read_in_worker=True,
+        )
+    )[0]
+
+
+def test_read_to_json(blank_nlp, tmpdir):
+    input_dir = Path(__file__).parent.parent.resolve() / "resources" / "docs.jsonl"
+    output_dir = Path(tmpdir)
+    doc = list(
+        edsnlp.data.read_json(
+            input_dir,
+            converter="omop",
+            span_attributes=["etat", "assertion"],
+            doc_attributes=["context_var"],
+        )
+    )[0]
+    assert_doc_read(doc)
+    doc.ents[0]._.etat = "test"
+
+    edsnlp.data.write_json(
+        [doc],
+        output_dir / "docs.jsonl",
+        converter="omop",
+        doc_attributes=["context_var"],
+        span_attributes=["etat", "assertion"],
+        span_getter=["ents", "sosy", "localisation", "anatomie", "pathologie"],
+        lines=True,
+    )
+
+    with pytest.raises(FileExistsError):
+        edsnlp.data.write_json(
+            [doc],
+            output_dir / "docs.jsonl",
+            converter="omop",
+            doc_attributes=["context_var"],
+            span_attributes=["etat", "assertion"],
+            span_getter=["ents", "sosy", "localisation", "anatomie", "pathologie"],
+        )
+
+    with open(output_dir / "docs.jsonl") as f:
+        exported_obj = json.loads(f.readlines()[0])
+    assert_doc_write(exported_obj)
+
+    with pytest.raises(FileExistsError):
+        edsnlp.data.write_json(
+            [doc],
+            output_dir,
+            converter="omop",
+            doc_attributes=["context_var"],
+            span_attributes=["etat", "assertion"],
+            span_getter=["ents", "sosy", "localisation", "anatomie", "pathologie"],
+            lines=False,
+        )
+
+    edsnlp.data.write_json(
+        [doc],
+        output_dir,
+        converter="omop",
+        doc_attributes=["context_var"],
+        span_attributes=["etat", "assertion"],
+        span_getter=["ents", "sosy", "localisation", "anatomie", "pathologie"],
+        lines=False,
+        overwrite=True,
+    )
+
+    with open(output_dir / "subfolder" / "doc-1.json") as f:
+        exported_obj = json.loads(f.read())
+    assert_doc_write(exported_obj)
+
+
+@pytest.mark.parametrize("num_cpu_workers", [0, 2])
+@pytest.mark.parametrize("shuffle", ["dataset"])
+def test_read_shuffle_loop(
+    num_cpu_workers: int,
+    shuffle: Literal["dataset", "fragment"],
+):
+    input_dir = Path(__file__).parent.parent.resolve() / "resources" / "docs.jsonl"
+    notes = (
+        edsnlp.data.read_json(
+            input_dir,
+            shuffle=shuffle,
+            seed=42,
+            loop=True,
+        )
+        .map(lambda x: x["note_id"])
+        .set_processing(num_cpu_workers=num_cpu_workers)
+    )
+    notes = list(islice(notes, 6))
+    assert notes == [
+        "subfolder/doc-2",
+        "subfolder/doc-1",
+        "subfolder/doc-3",
+        "subfolder/doc-3",
+        "subfolder/doc-2",
+        "subfolder/doc-1",
+    ]