flatbuffers_guide_use_java.html - third_party/flatbuffers - Git at Google

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 <div class="PageDoc"><div class="header">
   <div class="headertitle">
 <div class="title">Use in Java </div>  </div>
 </div><!--header-->
 <div class="contents">
 <div class="textblock"><p><a class="anchor" id="md_JavaUsage"></a></p>
 <h1><a class="anchor" id="autotoc_md82"></a>
 Before you get started</h1>
 <p>Before diving into the FlatBuffers usage in Java, it should be noted that the <a class="el" href="flatbuffers_guide_tutorial.html">Tutorial</a> page has a complete guide to general FlatBuffers usage in all of the supported languages (including Java). This page is designed to cover the nuances of FlatBuffers usage, specific to Java.</p>
 <p>You should also have read the <a class="el" href="flatbuffers_guide_building.html">Building</a> documentation to build <code>flatc</code> and should be familiar with <a class="el" href="flatbuffers_guide_using_schema_compiler.html">Using the schema compiler</a> and <a class="el" href="flatbuffers_guide_writing_schema.html">Writing a schema</a>.</p>
 <h1><a class="anchor" id="autotoc_md83"></a>
 FlatBuffers Java code location</h1>
 <p>The code for the FlatBuffers Java library can be found at <code>flatbuffers/java/com/google/flatbuffers</code>. You can browse the library on the <a href="https://github.com/google/flatbuffers/tree/master/
 java/com/google/flatbuffers">FlatBuffers GitHub page</a>.</p>
 <h1><a class="anchor" id="autotoc_md84"></a>
 Testing the FlatBuffers Java libraries</h1>
 <p>The code to test the libraries can be found at <code>flatbuffers/tests</code>.</p>
 <p>The test code for Java is located in <a href="https://github.com/google
 /flatbuffers/blob/master/tests/JavaTest.java">JavaTest.java</a>.</p>
 <p>To run the tests, use either <a href="https://github.com/google/
 flatbuffers/blob/master/tests/JavaTest.sh">JavaTest.sh</a> or <a href="https://github.com/
 google/flatbuffers/blob/master/tests/JavaTest.bat">JavaTest.bat</a>, depending on your operating system.</p>
 <p><em>Note: These scripts require that <a href="https://www.oracle.com/java/index.html">Java</a> is installed.</em></p>
 <h1><a class="anchor" id="autotoc_md85"></a>
 Using the FlatBuffers Java library</h1>
 <p><em>Note: See <a class="el" href="flatbuffers_guide_tutorial.html">Tutorial</a> for a more in-depth example of how to use FlatBuffers in Java.</em></p>
 <p>FlatBuffers supports reading and writing binary FlatBuffers in Java.</p>
 <p>To use FlatBuffers in your own code, first generate Java classes from your schema with the <code>--java</code> option to <code>flatc</code>. Then you can include both FlatBuffers and the generated code to read or write a FlatBuffer.</p>
 <p>For example, here is how you would read a FlatBuffer binary file in Java: First, import the library and generated code. Then, you read a FlatBuffer binary file into a <code>byte[]</code>. You then turn the <code>byte[]</code> into a <code>ByteBuffer</code>, which you pass to the <code>getRootAsMyRootType</code> function:</p>
 <div class="fragment"><div class="line"><span class="keyword">import</span> MyGame.Example.*;</div>
 <div class="line"><span class="keyword">import</span> com.google.flatbuffers.FlatBufferBuilder;</div>
 <div class="line"> </div>
 <div class="line"><span class="comment">// This snippet ignores exceptions for brevity.</span></div>
 <div class="line">File file = <span class="keyword">new</span> File(<span class="stringliteral">&quot;monsterdata_test.mon&quot;</span>);</div>
 <div class="line">RandomAccessFile f = <span class="keyword">new</span> RandomAccessFile(file, <span class="stringliteral">&quot;r&quot;</span>);</div>
 <div class="line"><span class="keywordtype">byte</span>[] data = <span class="keyword">new</span> <span class="keywordtype">byte</span>[(int)f.length()];</div>
 <div class="line">f.readFully(data);</div>
 <div class="line">f.close();</div>
 <div class="line"> </div>
 <div class="line">ByteBuffer bb = ByteBuffer.wrap(data);</div>
 <div class="line">Monster monster = Monster.getRootAsMonster(bb);</div>
 </div><!-- fragment --><p>Now you can access the data from the <code>Monster monster</code>:</p>
 <div class="fragment"><div class="line"><span class="keywordtype">short</span> hp = monster.hp();</div>
 <div class="line">Vec3 pos = monster.pos();</div>
 </div><!-- fragment --><h1><a class="anchor" id="autotoc_md86"></a>
 Storing dictionaries in a FlatBuffer</h1>
 <p>FlatBuffers doesn't support dictionaries natively, but there is support to emulate their behavior with vectors and binary search, which means you can have fast lookups directly from a FlatBuffer without having to unpack your data into a <code>Dictionary</code> or similar.</p>
 <p>To use it:</p><ul>
 <li>Designate one of the fields in a table as the "key" field. You do this by setting the <code>key</code> attribute on this field, e.g. <code>name:string (key)</code>. You may only have one key field, and it must be of string or scalar type.</li>
 <li>Write out tables of this type as usual, collect their offsets in an array.</li>
 <li>Instead of calling standard generated method, e.g.: <code>Monster.createTestarrayoftablesVector</code>, call <code>createSortedVectorOfTables</code> (from the <code>FlatBufferBuilder</code> object). which will first sort all offsets such that the tables they refer to are sorted by the key field, then serialize it.</li>
 <li>Now when you're accessing the FlatBuffer, you can use the <code>ByKey</code> accessor to access elements of the vector, e.g.: <code>monster.testarrayoftablesByKey("Frodo")</code>. which returns an object of the corresponding table type, or <code>null</code> if not found. <code>ByKey</code> performs a binary search, so should have a similar speed to <code>Dictionary</code>, though may be faster because of better caching. <code>ByKey</code> only works if the vector has been sorted, it will likely not find elements if it hasn't been sorted.</li>
 </ul>
 <h1><a class="anchor" id="autotoc_md87"></a>
 Text parsing</h1>
 <p>There currently is no support for parsing text (Schema's and JSON) directly from Java, though you could use the C++ parser through native call interfaces available to each language. Please see the C++ documentation for more on text parsing.</p>
 <p><br  />
  </p>
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	An open source project by <a href="https://developers.google.com/games/#Tools">FPL</a>.
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	<div class="PageDoc"><div class="header">
	<div class="headertitle">
	<div class="title">Use in Java </div> </div>
	</div><!--header-->
	<div class="contents">
	<div class="textblock"><p><a class="anchor" id="md_JavaUsage"></a></p>
	<h1><a class="anchor" id="autotoc_md82"></a>
	Before you get started</h1>
	<p>Before diving into the FlatBuffers usage in Java, it should be noted that the <a class="el" href="flatbuffers_guide_tutorial.html">Tutorial</a> page has a complete guide to general FlatBuffers usage in all of the supported languages (including Java). This page is designed to cover the nuances of FlatBuffers usage, specific to Java.</p>
	<p>You should also have read the <a class="el" href="flatbuffers_guide_building.html">Building</a> documentation to build <code>flatc</code> and should be familiar with <a class="el" href="flatbuffers_guide_using_schema_compiler.html">Using the schema compiler</a> and <a class="el" href="flatbuffers_guide_writing_schema.html">Writing a schema</a>.</p>
	<h1><a class="anchor" id="autotoc_md83"></a>
	FlatBuffers Java code location</h1>
	<p>The code for the FlatBuffers Java library can be found at <code>flatbuffers/java/com/google/flatbuffers</code>. You can browse the library on the <a href="https://github.com/google/flatbuffers/tree/master/
	java/com/google/flatbuffers">FlatBuffers GitHub page</a>.</p>
	<h1><a class="anchor" id="autotoc_md84"></a>
	Testing the FlatBuffers Java libraries</h1>
	<p>The code to test the libraries can be found at <code>flatbuffers/tests</code>.</p>
	<p>The test code for Java is located in <a href="https://github.com/google
	/flatbuffers/blob/master/tests/JavaTest.java">JavaTest.java</a>.</p>
	<p>To run the tests, use either <a href="https://github.com/google/
	flatbuffers/blob/master/tests/JavaTest.sh">JavaTest.sh</a> or <a href="https://github.com/
	google/flatbuffers/blob/master/tests/JavaTest.bat">JavaTest.bat</a>, depending on your operating system.</p>
	<p><em>Note: These scripts require that <a href="https://www.oracle.com/java/index.html">Java</a> is installed.</em></p>
	<h1><a class="anchor" id="autotoc_md85"></a>
	Using the FlatBuffers Java library</h1>
	<p><em>Note: See <a class="el" href="flatbuffers_guide_tutorial.html">Tutorial</a> for a more in-depth example of how to use FlatBuffers in Java.</em></p>
	<p>FlatBuffers supports reading and writing binary FlatBuffers in Java.</p>
	<p>To use FlatBuffers in your own code, first generate Java classes from your schema with the <code>--java</code> option to <code>flatc</code>. Then you can include both FlatBuffers and the generated code to read or write a FlatBuffer.</p>
	<p>For example, here is how you would read a FlatBuffer binary file in Java: First, import the library and generated code. Then, you read a FlatBuffer binary file into a <code>byte[]</code>. You then turn the <code>byte[]</code> into a <code>ByteBuffer</code>, which you pass to the <code>getRootAsMyRootType</code> function:</p>
	<div class="fragment"><div class="line"><span class="keyword">import</span> MyGame.Example.*;</div>
	<div class="line"><span class="keyword">import</span> com.google.flatbuffers.FlatBufferBuilder;</div>
	<div class="line"> </div>
	<div class="line"><span class="comment">// This snippet ignores exceptions for brevity.</span></div>
	<div class="line">File file = <span class="keyword">new</span> File(<span class="stringliteral">"monsterdata_test.mon"</span>);</div>
	<div class="line">RandomAccessFile f = <span class="keyword">new</span> RandomAccessFile(file, <span class="stringliteral">"r"</span>);</div>
	<div class="line"><span class="keywordtype">byte</span>[] data = <span class="keyword">new</span> <span class="keywordtype">byte</span>[(int)f.length()];</div>
	<div class="line">f.readFully(data);</div>
	<div class="line">f.close();</div>
	<div class="line"> </div>
	<div class="line">ByteBuffer bb = ByteBuffer.wrap(data);</div>
	<div class="line">Monster monster = Monster.getRootAsMonster(bb);</div>
	</div><!-- fragment --><p>Now you can access the data from the <code>Monster monster</code>:</p>
	<div class="fragment"><div class="line"><span class="keywordtype">short</span> hp = monster.hp();</div>
	<div class="line">Vec3 pos = monster.pos();</div>
	</div><!-- fragment --><h1><a class="anchor" id="autotoc_md86"></a>
	Storing dictionaries in a FlatBuffer</h1>
	<p>FlatBuffers doesn't support dictionaries natively, but there is support to emulate their behavior with vectors and binary search, which means you can have fast lookups directly from a FlatBuffer without having to unpack your data into a <code>Dictionary</code> or similar.</p>
	<p>To use it:</p><ul>
	<li>Designate one of the fields in a table as the "key" field. You do this by setting the <code>key</code> attribute on this field, e.g. <code>name:string (key)</code>. You may only have one key field, and it must be of string or scalar type.</li>
	<li>Write out tables of this type as usual, collect their offsets in an array.</li>
	<li>Instead of calling standard generated method, e.g.: <code>Monster.createTestarrayoftablesVector</code>, call <code>createSortedVectorOfTables</code> (from the <code>FlatBufferBuilder</code> object). which will first sort all offsets such that the tables they refer to are sorted by the key field, then serialize it.</li>
	<li>Now when you're accessing the FlatBuffer, you can use the <code>ByKey</code> accessor to access elements of the vector, e.g.: <code>monster.testarrayoftablesByKey("Frodo")</code>. which returns an object of the corresponding table type, or <code>null</code> if not found. <code>ByKey</code> performs a binary search, so should have a similar speed to <code>Dictionary</code>, though may be faster because of better caching. <code>ByKey</code> only works if the vector has been sorted, it will likely not find elements if it hasn't been sorted.</li>
	</ul>
	<h1><a class="anchor" id="autotoc_md87"></a>
	Text parsing</h1>
	<p>There currently is no support for parsing text (Schema's and JSON) directly from Java, though you could use the C++ parser through native call interfaces available to each language. Please see the C++ documentation for more on text parsing.</p>
	<p><br />
	</p>
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