forked from BigfootDev/flatbuffers
26a30738a4
Change-Id: I4c9f0f722490b374257adb3fec63e44ae93da920 Tested: using VS2010 / Xcode / gcc on Linux.
103 lines
5.8 KiB
HTML
103 lines
5.8 KiB
HTML
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<div id="projectname">FlatBuffers
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<div class="title">Use in Java </div> </div>
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<div class="textblock"><p>There's experimental support for reading FlatBuffers in Java. Generate code for Java with the <code>-j</code> option to <code>flatc</code>.</p>
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<p>See <code>javaTest.java</code> for an example. Essentially, you read a FlatBuffer binary file into a <code>byte[]</code>, which you then turn into a <code>ByteBuffer</code>, which you pass to the <code>getRootAsMonster</code> function: </p>
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<pre class="fragment">ByteBuffer bb = ByteBuffer.wrap(data);
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Monster monster = Monster.getRootAsMonster(bb);
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</pre><p>Now you can access values much like C++: </p>
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<pre class="fragment">short hp = monster.hp();
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Vec3 pos = monster.pos();
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</pre><p>Note that whenever you access a new object like in the <code>pos</code> 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 <code>pos()</code> method to which you can pass a <code>Vec3</code> 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.</p>
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<p>Sadly the string accessors currently always create a new string when accessed, since FlatBuffer's UTF-8 strings can't be read in-place by Java.</p>
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<p>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 <code>_length</code> let's you know the number of elements you can access: </p>
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<pre class="fragment">for (int i = 0; i < monster.inventory_length(); i++)
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monster.inventory(i); // do something here
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</pre><p>You can also construct these buffers in Java using the static methods found in the generated code, and the FlatBufferBuilder class: </p>
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<pre class="fragment">FlatBufferBuilder fbb = new FlatBufferBuilder();
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</pre><p>Create strings: </p>
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<pre class="fragment">int str = fbb.createString("MyMonster");
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</pre><p>Create a table with a struct contained therein: </p>
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<pre class="fragment">Monster.startMonster(fbb);
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Monster.addPos(fbb, Vec3.createVec3(fbb, 1.0f, 2.0f, 3.0f, 3.0, (byte)4, (short)5, (byte)6));
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Monster.addHp(fbb, (short)80);
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Monster.addName(fbb, str);
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Monster.addInventory(fbb, inv);
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Monster.addTest_type(fbb, (byte)1);
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Monster.addTest(fbb, mon2);
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Monster.addTest4(fbb, test4s);
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int mon = Monster.endMonster(fbb);
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</pre><p>As you can see, the Java code for tables does not use a convenient <code>createMonster</code> call like the C++ code. This is to create the buffer without using temporary object allocation (since the <code>Vec3</code> is an inline component of <code>Monster</code>, it has to be created right where it is added, whereas the name and the inventory are not inline). Structs do have convenient methods that even have arguments for nested structs.</p>
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<p>Vectors also use this start/end pattern to allow vectors of both scalar types and structs: </p>
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<pre class="fragment">Monster.startInventoryVector(fbb, 5);
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for (byte i = 4; i >=0; i--) fbb.addByte(i);
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int inv = fbb.endVector();
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</pre><p>You can use the generated method <code>startInventoryVector</code> to conveniently call <code>startVector</code> 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.</p>
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<h2>Text Parsing</h2>
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<p>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. </p>
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