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PojoSwaps

<p>
	{@link oaj.transform.PojoSwap PojoSwaps} are a critical component of Juneau.
	They allow the serializers and parsers to handle Java objects that wouldn't normally be serializable.
</p>
<p>
	Swaps are, simply put, 'object swappers' that swap in serializable objects for 
	non-serializable ones during serialization, and vis-versa during parsing.
</p>
<p>
	Some examples of non-serializable POJOs are <c>File</c>, <c>Reader</c>, 
	<c>Iterable</c>, etc...
	These are classes that aren't beans and cannot be represented as simple maps, collections, or primitives.
</p>
<p>
	In the following example, we introduce a <c>PojoSwap</c> that will swap in ISO8601 strings for 
	<c>Date</c> objects:
</p>
<p class='bpcode w800'>
	<jc>// Sample swap for converting Dates to ISO8601 strings.</jc>
	<jk>public class</jk> MyDateSwap <jk>extends</jk> PojoSwap&lt;Date,String&gt; {
		
		<jc>// ISO8601 formatter.</jc>
		<jk>private</jk> DateFormat <jf>format</jf> = <jk>new</jk> SimpleDateFormat(<js>"yyyy-MM-dd'T'HH:mm:ssZ"</js>);
		
		<jc>// Converts a Date object to an ISO8601 string.</jc>
		<ja>@Override</ja> <jc>/* PojoSwap */</jc>
		<jk>public</jk> String swap(BeanSession session, Date o) {
			<jk>return</jk> <jf>format</jf>.format(o);
		}
		
		<jc>// Converts an ISO8601 string to a Date object.</jc>
		<ja>@Override</ja> <jc>/* PojoSwap */</jc>
		<jk>public</jk> Date unswap(BeanSession session, String o, ClassMeta hint) <jk>throws</jk> Exception {
			<jk>return</jk> <jf>format</jf>.parse(o);
		}
	}
</p>
<p>
	The swap can then be associated with serializers and parsers like so:
</p>
<p class='bpcode w800'>
	<jc>// Sample bean with a Date field.</jc>
	<jk>public class</jk> MyBean {
		<jk>public</jk> Date <jf>date</jf> = <jk>new</jk> Date(112, 2, 3, 4, 5, 6);
	}

	<jc>// Create a new JSON serializer, associate our date swap with it, and serialize a sample bean.</jc>
	WriterSerializer s = JsonSerializer.<jsm>create</jsm>().simple().pojoSwaps(MyDateSwap.<jk>class</jk>).build();
	String json = s.serialize(<jk>new</jk> MyBean());	<jc>// == "{date:'2012-03-03T04:05:06-0500'}"</jc>
	
	<jc>// Create a JSON parser, associate our date swap with it, and reconstruct our bean (including the date).</jc>
	ReaderParser p = JsonParser.<jsm>create</jsm>().pojoSwaps(MyDateSwap.<jk>class</jk>).build();
	MyBean bean = p.parse(json, MyBean.<jk>class</jk>);
	<jk>int</jk> day = bean.<jf>date</jf>.getDay(); 						<jc>// == 3</jc>
</p>
<p>
	The {@link oaj.BeanMap#get(Object)} and {@link oaj.BeanMap#put(String,Object)} 
	methods will automatically convert to swapped values as the following example shows:
</p>
<p class='bpcode w800'>
	<jc>// Create a new bean context and add our swap.</jc>
	BeanContext bc = BeanContext.<jsm>create</jsm>().pojoSwaps(MyDateSwap.<jk>class</jk>).build();

	<jc>// Create a new bean.</jc>
	MyBean myBean = <jk>new</jk> MyBean();

	<jc>// Wrap it in a bean map.</jc>
	BeanMap&lt;Bean&gt; beanMap = bc.forBean(myBean);

	<jc>// Use the get() method to get the date field as an ISO8601 string.</jc>
	String date = (String)beanMap.get(<js>"date"</js>);				<jc>// == "2012-03-03T04:05:06-0500"</jc> 
	
	<jc>// Use the put() method to set the date field to an ISO8601 string.</jc>
	beanMap.put(<js>"date"</js>, <js>"2013-01-01T12:30:00-0500"</js>);	<jc>// Set it to a new value.</jc> 
	
	<jc>// Verify that the date changed on the original bean.</jc>
	<jk>int</jk> year = myBean.<jf>date</jf>.getYear(); 								<jc>// == 113</jc>
</p>
<p>
	Another example of a <c>PojoSwap</c> is one that converts <c><jk>byte</jk>[]</c> arrays to
	BASE64-encoded strings:
</p>
<p class='bpcode w800'>
	<jk>public class</jk> ByteArrayBase64Swap <jk>extends</jk> StringSwap&lt;<jk>byte</jk>[]&gt; {
	
		<ja>@Override</ja> <jc>/* StringSwap */</jc>
		<jk>public</jk> String swap(<jk>byte</jk>[] b) <jk>throws</jk> Exception {
			ByteArrayOutputStream baos = <jk>new</jk> ByteArrayOutputStream();
			OutputStream b64os = MimeUtility.encode(baos, <js>"base64"</js>);
			b64os.write(b);
			b64os.close();
			<jk>return new</jk> String(baos.toByteArray());
		}
		
		<ja>@Override</ja> <jc>/* StringSwap */</jc>
		<jk>public byte</jk>[] unswap(String s, ClassMeta&lt;?&gt; hint) <jk>throws</jk> Exception {
			<jk>byte</jk>[] b = s.getBytes();
			ByteArrayInputStream bais = <jk>new</jk> ByteArrayInputStream(b);
			InputStream b64is = MimeUtility.decode(bais, <js>"base64"</js>);
			<jk>byte</jk>[] tmp = <jk>new byte</jk>[b.length];
			<jk>int</jk> n = b64is.read(tmp);
			<jk>byte</jk>[] res = <jk>new byte</jk>[n];
			System.<jsm>arraycopy</jsm>(tmp, 0, res, 0, n);
			<jk>return</jk> res;
		}
	}
</p>
<p>
	The following example shows the BASE64 swap in use:
</p>
<p class='bpcode w800'>
	<jc>// Create a JSON serializer and register the BASE64 encoding swap with it.</jc>
	WriterSerializer s = JsonSerializer.<jsm>create</jsm>().simple().pojoSwaps(ByteArrayBase64Swap.<jk>class</jk>).build();
	ReaderParser p = JsonParser.<jsm>create</jsm>().pojoSwaps(ByteArrayBase64Swap.<jk>class</jk>).build();
	
	<jk>byte</jk>[] bytes = {1,2,3};
	String json = s.serialize(bytes);		<jc>// Produces "'AQID'"</jc>
	bytes = p.parse(json, <jk>byte</jk>[].<jk>class</jk>);		<jc>// Reproduces {1,2,3}</jc>
	
	<jk>byte</jk>[][] bytes2d = {{1,2,3},{4,5,6},<jk>null</jk>};
	json = s.serialize(bytes2d);		<jc>// Produces "['AQID','BAUG',null]"</jc>
	bytes2d = p.parse(json, <jk>byte</jk>[][].<jk>class</jk>);		<jc>// Reproduces {{1,2,3},{4,5,6},null}</jc>
</p>
<p>
	Several <c>PojoSwaps</c> are already provided for common Java objects:
</p>
<ul class='doctree'>
	<li class='jp'><jk>org.apache.juneau.transforms</jk>
		<ul>
			<li class='jc'>
				{@link oaj.transforms.ByteArrayBase64Swap}
			<li class='jac'>
				{@link oaj.transforms.CalendarSwap}
			<li class='jac'>
				{@link oaj.transforms.DateSwap}
			<li class='jc'>
				{@link oaj.transforms.EnumerationSwap}
			<li class='jc'>
				{@link oaj.transforms.IteratorSwap}
			<li class='jc'>
				{@link oaj.transforms.ReaderSwap}
			<li class='jc'>
				{@link oaj.transforms.XMLGregorianCalendarSwap}
		</ul>
	</li>
</ul>
<p>
	In particular, the {@link oaj.transforms.CalendarSwap} and 
	{@link oaj.transforms.DateSwap} transforms provide a large number of customized swaps to 
	ISO, RFC, or localized strings.
</p>
<p>
	Swaps have access to the session locale and timezone through the {@link oaj.BeanSession#getLocale()} and
	{@link oaj.BeanSession#getTimeZone()} methods.
	This allows you to specify localized swap values when needed.
</p>
<p>
	If using the REST server API, the locale and timezone are set based on the <c>Accept-Language</c> and
	<c>Time-Zone</c> headers on the request.
</p>

<ul class='doctree'>
	<li class='info'>
		The 'swapped' class type must be a serializable type.
		<br>See the definition for Category 4 objects in {@doc PojoCategories}.  
</ul>