flatbuffers/docs/source/JavaUsage.md

Use in Java

FlatBuffers supports reading and writing binary FlatBuffers in Java. Generate code for Java with the -j option to flatc.

See javaTest.java for an example. Essentially, you read a FlatBuffer binary file into a byte[], which you then turn into a ByteBuffer, which you pass to the getRootAsMyRootType function:

ByteBuffer bb = ByteBuffer.wrap(data);
Monster monster = Monster.getRootAsMonster(bb);

Now you can access values much like C++:

short hp = monster.hp();
Vec3 pos = monster.pos();

Note that whenever you access a new object like in the pos example above, a new temporary accessor object gets created. If your code is very performance sensitive (you iterate through a lot of objects), there's a second pos() method to which you can pass a Vec3 object you've already created. This allows you to reuse it across many calls and reduce the amount of object allocation (and thus garbage collection) your program does.

Java does not support unsigned scalars. This means that any unsigned types you use in your schema will actually be represented as a signed value. This means all bits are still present, but may represent a negative value when used. For example, to read a byte b as an unsigned number, you can do: (short)(b & 0xFF)

The default string accessor (e.g. monster.name()) currently always create a new Java String when accessed, since FlatBuffer's UTF-8 strings can't be used in-place by String. Alternatively, use monster.nameAsByteBuffer() which returns a ByteBuffer referring to the UTF-8 data in the original ByteBuffer, which is much more efficient. The ByteBuffer's position points to the first character, and its limit to just after the last.

Vector access is also a bit different from C++: you pass an extra index to the vector field accessor. Then a second method with the same name suffixed by Length let's you know the number of elements you can access:

for (int i = 0; i < monster.inventoryLength(); i++)
    monster.inventory(i); // do something here

Alternatively, much like strings, you can use monster.inventoryAsByteBuffer() to get a ByteBuffer referring to the whole vector. Use ByteBuffer methods like asFloatBuffer to get specific views if needed.

If you specified a file_indentifier in the schema, you can query if the buffer is of the desired type before accessing it using:

if (Monster.MonsterBufferHasIdentifier(bb)) ...

Buffer construction in Java

You can also construct these buffers in Java using the static methods found in the generated code, and the FlatBufferBuilder class:

FlatBufferBuilder fbb = new FlatBufferBuilder();

Create strings:

int str = fbb.createString("MyMonster");

Create a table with a struct contained therein:

Monster.startMonster(fbb);
Monster.addPos(fbb, Vec3.createVec3(fbb, 1.0f, 2.0f, 3.0f, 3.0, (byte)4, (short)5, (byte)6));
Monster.addHp(fbb, (short)80);
Monster.addName(fbb, str);
Monster.addInventory(fbb, inv);
Monster.addTest_type(fbb, (byte)1);
Monster.addTest(fbb, mon2);
Monster.addTest4(fbb, test4s);
int mon = Monster.endMonster(fbb);

As you can see, the Java code for tables does not use a convenient createMonster call like the C++ code. This is to create the buffer without using temporary object allocation.

It's important to understand that fields that are structs are inline (like Vec3 above), and MUST thus be created between the start and end calls of a table. Everything else (other tables, strings, vectors) MUST be created before the start of the table they are referenced in.

Structs do have convenient methods that even have arguments for nested structs.

As you can see, references to other objects (e.g. the string above) are simple ints, and thus do not have the type-safety of the Offset type in C++. Extra case must thus be taken that you set the right offset on the right field.

Vectors also use this start/end pattern to allow vectors of both scalar types and structs:

Monster.startInventoryVector(fbb, 5);
for (byte i = 4; i >=0; i--) fbb.addByte(i);
int inv = fbb.endVector();

You can use the generated method startInventoryVector to conveniently call startVector with the right element size. You pass the number of elements you want to write. You write the elements backwards since the buffer is being constructed back to front.

There are add functions for all the scalar types. You use addOffset for any previously constructed objects (such as other tables, strings, vectors). For structs, you use the appropriate create function in-line, as shown above in the Monster example.

To finish the buffer, call:

Monster.finishMonsterBuffer(fbb, mon);

The buffer is now ready to be transmitted. It is contained in the ByteBuffer which you can obtain from fbb.dataBuffer(). Importantly, the valid data does not start from offset 0 in this buffer, but from fbb.dataBuffer().position() (this is because the data was built backwards in memory). It ends at fbb.capacity().

Text Parsing

There currently is no support for parsing text (Schema's and JSON) directly from Java, though you could use the C++ parser through JNI. Please see the C++ documentation for more on text parsing.