[api] Support encode/decode String tensor

api/src/main/java/ai/djl/ndarray/NDSerializer.java

@@ -83,6 +83,16 @@ static void encode(NDArray array, OutputStream os) throws IOException {
         Shape shape = array.getShape();
         dos.write(shape.getEncoded());
 
+        if (array.getDataType() == DataType.STRING) {
+            String[] data = array.toStringArray();
+            dos.writeInt(data.length);
+            for (String str : data) {
+                dos.writeUTF(str);
+            }
+            dos.flush();
+            return;
+        }
+
         ByteBuffer bb = array.toByteBuffer();
         dos.write(bb.order() == ByteOrder.BIG_ENDIAN ? '>' : '<');
         int length = bb.remaining();
@@ -167,6 +177,17 @@ static NDArray decode(NDManager manager, ByteBuffer bb) {
         // Shape
         Shape shape = Shape.decode(bb);
 
+        if (dataType == DataType.STRING) {
+            int size = bb.getInt();
+            String[] data = new String[size];
+            for (int i = 0; i < size; ++i) {
+                data[i] = readUTF(bb);
+            }
+            NDArray array = manager.create(data, StandardCharsets.UTF_8, shape);
+            array.setName(name);
+            return array;
+        }
+
         // Data
         ByteOrder order;
         if (version > 2) {

api/src/test/java/ai/djl/ndarray/NDSerializerTest.java

@@ -97,6 +97,23 @@ public void testNDSerializer() throws IOException {
         }
     }
 
+    @Test
+    public void testStringTensor() {
+        try (NDManager manager = NDManager.newBaseManager("PyTorch")) {
+            NDArray array = manager.create("hello");
+            byte[] buf = array.encode();
+            NDArray decoded = NDArray.decode(manager, buf);
+            Assert.assertTrue(decoded.getShape().isScalar());
+
+            array = manager.create(new String[] {"hello", "world"});
+            buf = array.encode();
+            decoded = NDArray.decode(manager, buf);
+            Assert.assertEquals(decoded.getShape(), array.getShape());
+            Assert.assertEquals(decoded.toStringArray()[1], "world");
+            Assert.assertEquals(decoded, array);
+        }
+    }
+
     private static byte[] encode(NDArray array) throws IOException {
         try (ByteArrayOutputStream bos = new ByteArrayOutputStream()) {
             NDSerializer.encodeAsNumpy(array, bos);

engines/pytorch/pytorch-engine/src/main/java/ai/djl/pytorch/engine/PtNDArray.java

@@ -100,6 +100,7 @@ public PtNDArray(PtNDManager manager, String[] strs, Shape shape) {
         super(-1L);
         this.manager = manager;
         this.strs = strs;
+        this.sparseFormat = SparseFormat.DENSE;
         this.shape = shape;
         this.dataType = DataType.STRING;
         NDScope.register(this);
@@ -225,6 +226,10 @@ public NDArray stopGradient() {
     /** {@inheritDoc} */
     @Override
     public ByteBuffer toByteBuffer() {
+        if (getDataType() == DataType.STRING) {
+            throw new UnsupportedOperationException(
+                    "toByteBuffer is not supported for String tensor.");
+        }
         return JniUtils.getByteBuffer(this);
     }
 
@@ -429,6 +434,9 @@ public boolean contentEquals(NDArray other) {
         if (getDataType() != other.getDataType()) {
             return false;
         }
+        if (getDataType() == DataType.STRING) {
+            return Arrays.equals(toStringArray(), other.toStringArray());
+        }
         return JniUtils.contentEqual(this, manager.from(other));
     }