In Java, allow reusing ByteBuffer in getters (#4633)

* In Java, allow reusing ByteBuffer in getters

* In Java, allow reusing ByteBuffer in getters

* In Java, allow reusing ByteBuffer in getters

java/com/google/flatbuffers/Table.java

@@ -172,6 +172,27 @@ public class Table {
     return bb;
   }
 
+  /**
+   * Initialize vector as a ByteBuffer.
+   *
+   * This is more efficient than using duplicate, since it doesn't copy the data
+   * nor allocattes a new {@link ByteBuffer}, creating no garbage to be collected.
+   *
+   * @param bb The {@link ByteBuffer} for the array
+   * @param vector_offset The position of the vector in the byte buffer
+   * @param elem_size The size of each element in the array
+   * @return The {@link ByteBuffer} for the array
+   */
+  protected ByteBuffer __vector_in_bytebuffer(ByteBuffer bb, int vector_offset, int elem_size) {
+    int o = this.__offset(vector_offset);
+    if (o == 0) return null;
+    int vectorstart = __vector(o);
+    bb.rewind();
+    bb.limit(vectorstart + __vector_len(o) * elem_size);
+    bb.position(vectorstart);
+    return bb;
+  }
+
   /**
    * Initialize any Table-derived type to point to the union at the given `offset`.
    *

src/idl_gen_general.cpp

@@ -1066,6 +1066,16 @@ class GeneralGenerator : public BaseGenerator {
                                 ? 1
                                 : InlineSize(field.value.type.VectorType()));
             code += "); }\n";
+            code += "  public ByteBuffer ";
+            code += MakeCamel(field.name, lang_.first_camel_upper);
+            code += "InByteBuffer(ByteBuffer _bb) { return ";
+            code += lang_.accessor_prefix + "__vector_in_bytebuffer(_bb, ";
+            code += NumToString(field.value.offset) + ", ";
+            code +=
+                NumToString(field.value.type.base_type == BASE_TYPE_STRING
+                                ? 1
+                                : InlineSize(field.value.type.VectorType()));
+            code += "); }\n";
             break;
           case IDLOptions::kCSharp:
             code += "  public ArraySegment<byte>? Get";

tests/MyGame/Example/Monster.java

@@ -26,9 +26,11 @@ public final class Monster extends Table {
   public boolean mutateHp(short hp) { int o = __offset(8); if (o != 0) { bb.putShort(o + bb_pos, hp); return true; } else { return false; } }
   public String name() { int o = __offset(10); return o != 0 ? __string(o + bb_pos) : null; }
   public ByteBuffer nameAsByteBuffer() { return __vector_as_bytebuffer(10, 1); }
+  public ByteBuffer nameInByteBuffer(ByteBuffer _bb) { return __vector_in_bytebuffer(_bb, 10, 1); }
   public int inventory(int j) { int o = __offset(14); return o != 0 ? bb.get(__vector(o) + j * 1) & 0xFF : 0; }
   public int inventoryLength() { int o = __offset(14); return o != 0 ? __vector_len(o) : 0; }
   public ByteBuffer inventoryAsByteBuffer() { return __vector_as_bytebuffer(14, 1); }
+  public ByteBuffer inventoryInByteBuffer(ByteBuffer _bb) { return __vector_in_bytebuffer(_bb, 14, 1); }
   public boolean mutateInventory(int j, int inventory) { int o = __offset(14); if (o != 0) { bb.put(__vector(o) + j * 1, (byte)inventory); return true; } else { return false; } }
   public byte color() { int o = __offset(16); return o != 0 ? bb.get(o + bb_pos) : 8; }
   public boolean mutateColor(byte color) { int o = __offset(16); if (o != 0) { bb.put(o + bb_pos, color); return true; } else { return false; } }
@@ -53,6 +55,7 @@ public final class Monster extends Table {
   public int testnestedflatbuffer(int j) { int o = __offset(30); return o != 0 ? bb.get(__vector(o) + j * 1) & 0xFF : 0; }
   public int testnestedflatbufferLength() { int o = __offset(30); return o != 0 ? __vector_len(o) : 0; }
   public ByteBuffer testnestedflatbufferAsByteBuffer() { return __vector_as_bytebuffer(30, 1); }
+  public ByteBuffer testnestedflatbufferInByteBuffer(ByteBuffer _bb) { return __vector_in_bytebuffer(_bb, 30, 1); }
   public Monster testnestedflatbufferAsMonster() { return testnestedflatbufferAsMonster(new Monster()); }
   public Monster testnestedflatbufferAsMonster(Monster obj) { int o = __offset(30); return o != 0 ? obj.__assign(__indirect(__vector(o)), bb) : null; }
   public boolean mutateTestnestedflatbuffer(int j, int testnestedflatbuffer) { int o = __offset(30); if (o != 0) { bb.put(__vector(o) + j * 1, (byte)testnestedflatbuffer); return true; } else { return false; } }
@@ -79,6 +82,7 @@ public final class Monster extends Table {
   public boolean testarrayofbools(int j) { int o = __offset(52); return o != 0 ? 0!=bb.get(__vector(o) + j * 1) : false; }
   public int testarrayofboolsLength() { int o = __offset(52); return o != 0 ? __vector_len(o) : 0; }
   public ByteBuffer testarrayofboolsAsByteBuffer() { return __vector_as_bytebuffer(52, 1); }
+  public ByteBuffer testarrayofboolsInByteBuffer(ByteBuffer _bb) { return __vector_in_bytebuffer(_bb, 52, 1); }
   public boolean mutateTestarrayofbools(int j, boolean testarrayofbools) { int o = __offset(52); if (o != 0) { bb.put(__vector(o) + j * 1, (byte)(testarrayofbools ? 1 : 0)); return true; } else { return false; } }
   public float testf() { int o = __offset(54); return o != 0 ? bb.getFloat(o + bb_pos) : 3.14159f; }
   public boolean mutateTestf(float testf) { int o = __offset(54); if (o != 0) { bb.putFloat(o + bb_pos, testf); return true; } else { return false; } }
@@ -94,6 +98,7 @@ public final class Monster extends Table {
   public int flex(int j) { int o = __offset(64); return o != 0 ? bb.get(__vector(o) + j * 1) & 0xFF : 0; }
   public int flexLength() { int o = __offset(64); return o != 0 ? __vector_len(o) : 0; }
   public ByteBuffer flexAsByteBuffer() { return __vector_as_bytebuffer(64, 1); }
+  public ByteBuffer flexInByteBuffer(ByteBuffer _bb) { return __vector_in_bytebuffer(_bb, 64, 1); }
   public boolean mutateFlex(int j, int flex) { int o = __offset(64); if (o != 0) { bb.put(__vector(o) + j * 1, (byte)flex); return true; } else { return false; } }
   public Test test5(int j) { return test5(new Test(), j); }
   public Test test5(Test obj, int j) { int o = __offset(66); return o != 0 ? obj.__assign(__vector(o) + j * 4, bb) : null; }
@@ -101,10 +106,12 @@ public final class Monster extends Table {
   public long vectorOfLongs(int j) { int o = __offset(68); return o != 0 ? bb.getLong(__vector(o) + j * 8) : 0; }
   public int vectorOfLongsLength() { int o = __offset(68); return o != 0 ? __vector_len(o) : 0; }
   public ByteBuffer vectorOfLongsAsByteBuffer() { return __vector_as_bytebuffer(68, 8); }
+  public ByteBuffer vectorOfLongsInByteBuffer(ByteBuffer _bb) { return __vector_in_bytebuffer(_bb, 68, 8); }
   public boolean mutateVectorOfLongs(int j, long vector_of_longs) { int o = __offset(68); if (o != 0) { bb.putLong(__vector(o) + j * 8, vector_of_longs); return true; } else { return false; } }
   public double vectorOfDoubles(int j) { int o = __offset(70); return o != 0 ? bb.getDouble(__vector(o) + j * 8) : 0; }
   public int vectorOfDoublesLength() { int o = __offset(70); return o != 0 ? __vector_len(o) : 0; }
   public ByteBuffer vectorOfDoublesAsByteBuffer() { return __vector_as_bytebuffer(70, 8); }
+  public ByteBuffer vectorOfDoublesInByteBuffer(ByteBuffer _bb) { return __vector_in_bytebuffer(_bb, 70, 8); }
   public boolean mutateVectorOfDoubles(int j, double vector_of_doubles) { int o = __offset(70); if (o != 0) { bb.putDouble(__vector(o) + j * 8, vector_of_doubles); return true; } else { return false; } }
   public MyGame.InParentNamespace parentNamespaceTest() { return parentNamespaceTest(new MyGame.InParentNamespace()); }
   public MyGame.InParentNamespace parentNamespaceTest(MyGame.InParentNamespace obj) { int o = __offset(72); return o != 0 ? obj.__assign(__indirect(o + bb_pos), bb) : null; }
@@ -117,6 +124,7 @@ public final class Monster extends Table {
   public long vectorOfWeakReferences(int j) { int o = __offset(78); return o != 0 ? bb.getLong(__vector(o) + j * 8) : 0; }
   public int vectorOfWeakReferencesLength() { int o = __offset(78); return o != 0 ? __vector_len(o) : 0; }
   public ByteBuffer vectorOfWeakReferencesAsByteBuffer() { return __vector_as_bytebuffer(78, 8); }
+  public ByteBuffer vectorOfWeakReferencesInByteBuffer(ByteBuffer _bb) { return __vector_in_bytebuffer(_bb, 78, 8); }
   public boolean mutateVectorOfWeakReferences(int j, long vector_of_weak_references) { int o = __offset(78); if (o != 0) { bb.putLong(__vector(o) + j * 8, vector_of_weak_references); return true; } else { return false; } }
 
   public static void startMonster(FlatBufferBuilder builder) { builder.startObject(38); }