1Use in Java {#flatbuffers_guide_use_java}
2==============
3
4## Before you get started
5
6Before diving into the FlatBuffers usage in Java, it should be noted that
7the [Tutorial](@ref flatbuffers_guide_tutorial) page has a complete guide to
8general FlatBuffers usage in all of the supported languages (including Java).
9This page is designed to cover the nuances of FlatBuffers usage,
10specific to Java.
11
12You should also have read the [Building](@ref flatbuffers_guide_building)
13documentation to build `flatc` and should be familiar with
14[Using the schema compiler](@ref flatbuffers_guide_using_schema_compiler) and
15[Writing a schema](@ref flatbuffers_guide_writing_schema).
16
17## FlatBuffers Java code location
18
19The code for the FlatBuffers Java library can be found at
20`flatbuffers/java/com/google/flatbuffers`. You can browse the library on the
21[FlatBuffers GitHub page](https://github.com/google/flatbuffers/tree/master/
22java/com/google/flatbuffers).
23
24## Testing the FlatBuffers Java libraries
25
26The code to test the libraries can be found at `flatbuffers/tests`.
27
28The test code for Java is located in [JavaTest.java](https://github.com/google
29/flatbuffers/blob/master/tests/JavaTest.java).
30
31To run the tests, use either [JavaTest.sh](https://github.com/google/
32flatbuffers/blob/master/tests/JavaTest.sh) or [JavaTest.bat](https://github.com/
33google/flatbuffers/blob/master/tests/JavaTest.bat), depending on your operating
34system.
35
36*Note: These scripts require that [Java](https://www.oracle.com/java/index.html)
37is installed.*
38
39## Using the FlatBuffers Java library
40
41*Note: See [Tutorial](@ref flatbuffers_guide_tutorial) for a more in-depth
42example of how to use FlatBuffers in Java.*
43
44FlatBuffers supports reading and writing binary FlatBuffers in Java.
45
46To use FlatBuffers in your own code, first generate Java classes from your
47schema with the `--java` option to `flatc`.
48Then you can include both FlatBuffers and the generated code to read
49or write a FlatBuffer.
50
51For example, here is how you would read a FlatBuffer binary file in Java:
52First, import the library and generated code. Then, you read a FlatBuffer binary
53file into a `byte[]`. You then turn the `byte[]` into a `ByteBuffer`, which you
54pass to the `getRootAsMyRootType` function:
55
56~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.java}
57 import MyGame.Example.*;
58 import com.google.flatbuffers.FlatBufferBuilder;
59
60 // This snippet ignores exceptions for brevity.
61 File file = new File("monsterdata_test.mon");
62 RandomAccessFile f = new RandomAccessFile(file, "r");
63 byte[] data = new byte[(int)f.length()];
64 f.readFully(data);
65 f.close();
66
67 ByteBuffer bb = ByteBuffer.wrap(data);
68 Monster monster = Monster.getRootAsMonster(bb);
69~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
70
71Now you can access the data from the `Monster monster`:
72
73~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.java}
74 short hp = monster.hp();
75 Vec3 pos = monster.pos();
76~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
77
78## Storing dictionaries in a FlatBuffer
79
80FlatBuffers doesn't support dictionaries natively, but there is support to
81emulate their behavior with vectors and binary search, which means you
82can have fast lookups directly from a FlatBuffer without having to unpack
83your data into a `Dictionary` or similar.
84
85To use it:
86- Designate one of the fields in a table as the "key" field. You do this
87 by setting the `key` attribute on this field, e.g.
88 `name:string (key)`.
89 You may only have one key field, and it must be of string or scalar type.
90- Write out tables of this type as usual, collect their offsets in an
91 array.
92- Instead of calling standard generated method,
93 e.g.: `Monster.createTestarrayoftablesVector`,
94 call `createSortedVectorOfTables` (from the `FlatBufferBuilder` object).
95 which will first sort all offsets such that the tables they refer to
96 are sorted by the key field, then serialize it.
97- Now when you're accessing the FlatBuffer, you can use
98 the `ByKey` accessor to access elements of the vector, e.g.:
99 `monster.testarrayoftablesByKey("Frodo")`.
100 which returns an object of the corresponding table type,
101 or `null` if not found.
102 `ByKey` performs a binary search, so should have a similar
103 speed to `Dictionary`, though may be faster because of better caching.
104 `ByKey` only works if the vector has been sorted, it will
105 likely not find elements if it hasn't been sorted.
106
107## Text parsing
108
109There currently is no support for parsing text (Schema's and JSON) directly
110from Java, though you could use the C++ parser through native call
111interfaces available to each language. Please see the
112C++ documentation for more on text parsing.
113
114<br>
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