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{{Mark|Updated with Version 8.4}}: A Java class is identified by a namespace URI. The original URI is rewritten as follows:
A Java variable is retrieved, functions and a Java function is invoked, variables can be referenced and evaluated by a usual the existing XQuery function call. The namespace of its QName identifies the class, and the local part, which is rewritten to camel case, identifies a variable or function of that classsyntax:
[[Category:XQuery]]
[[Category:API]]
→Context-Awareness
The Java Binding feature is an extensibility mechanism which enables developers
to directly access Java variables and execute code from XQuery. Addressed Java code must either be contained in the Java classpath, or it must be located in the [[Repository]].
Please bear in mind that the execution of Java code may cause side effects that conflict with the functional nature of XQuery, or may introduce new security risks to your project.
=Identification=
==Classes==
# The [[Repository#URI_Rewriting|URI Rewriting]] steps are applied to the URI.
# The last path segment of the URI is capitalized and rewritten to [https://en.wikipedia.org/wiki/CamelCase camel case].
The normalization steps are skipped if the URI is prefixed with {{Code|java:}}. See the following examples:
* <code><nowiki>http://basex.org/modules/meta-data</nowiki></code> → <code>org.basex.modules.MetaData</code>
==Functions and Variables==
* The namespace of the function name identifies the Java class.* The local part of the name, which is rewritten to camel case, identifies a variable or function of that class.* The middle dot character <code>·</code> ([http://www.fileformat.info/info/unicode/char/b7/index.htm &#xB7;], a valid character in XQuery names, but not in Java) can be used to append exact Java parameter types to the function name. Class types must be referenced by their full path. {| class="wikitable"|- valign="top"! Type! XQuery! Java|- valign="top"| Variable| <code>Q{java.lang.Integer}MIN_VALUE()</code> → | <code>[https://docs.oracle.com/javase/8/docs/api/java/lang/Integer.html#MAX_VALUE Integer.MIN_VALUE]</code><br/> * |- valign="top"| Function| <code>Q{java.lang.Object}hash-code()</code> → | <code>[https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#hashCode()object.hashCode()]</code>|- valign="top"| Function with types| <code>Q{java.lang.String}split·java.lang.String·int(';', 3)</code>| <code>[https://docs.oracle.com/javase/8/docs/api/java/lang/String.html#split-java.lang.String-int- string.split(";", 3)]<br/code> |} As XQuery and Java have different type systems, XQuery arguments are converted to equivalent Java values, and the result of a Java function is converted back to an XQuery value (see [[#Data Types|Data Types]]). If a Java function is not found, XQuery values may need to be cast the target type. For example, if a Java function expects a primitive {{Code|int}} value, you will need to convert your XQuery integers to {{Code|xs:int}}.
=Namespace Declarations=
</pre>
The constructor of a class can be invoked by calling the virtual function {{Code|new()}}. Instance methods can then called by passing on the resulting Java object as first argument. In the following example, 256 bytes are written to the file {{Code|output.txt}}. First, a new {{Code|FileWriter}} instance is created, and its {{Code|write()}} function is called in the next step:
<pre class="brush:xquery">declare namespace fw = "java.io.FileWriter";
=Module Imports=
Java code can also be integrated accessed by ''importing'' classes as modules. A new instance of the addressed class is will be created, which can then be accessed referenced in the query body.
The following, (side-effecting ) example returns the number of distinct values added to a hash set (the . The boolean values returned by {{Code|set:add()}} will be swallowed):
<pre class="brush:xquery">
import module namespace set = "java:java.util.HashSet";
prof:void(
for $s in ("one", "two", "one")
</pre>
The advantages of this approach is that the execution of imported code classes is executed faster more efficient than the execution of instances that are created at runtime via {{Code|new()}}. A drawback is that no arguments can be passed on to the class constructor. As a consequence, the import only works fails if the addressed class provides a has no default constructor, but at least one constructor with no arguments.
=Context-Awareness=
Java classes can be coupled even more closely to the BaseX core library.
If a class inherits the abstract [https://github.com/BaseXdb/basex/blob/master/basex-core/src/main/java/org/basex/query/QueryModule.java QueryModule] class, the two variables [https://github.com/BaseXdb/basex/blob/master/basex-core/src/main/java/org/basex/query/QueryContext.java queryContext] and [https://github.com/BaseXdb/basex/blob/master/basex-core/src/main/java/org/basex/query/StaticContext.java staticContext] get available, which provide access to the global and static context of a query. Additionally, the default properties of functions can be changed via annotations:
If an XQuery expression is run which calls the Java {{Code|write()}} function, every other query that calls {{Code|write()}} or {{Code|read()}} needs to wait for the query to be finished. If a query calls the {{Code|read()}} function, only those queries are queued that call {{Code|write()}}, because this function is only annotated with a {{Code|read}} lock. More details on parallel query execution can be found in the article on [[Transaction Management]].
=Data Types=
The following table lists the mappings of XQuery and Java types:
{| class="wikitable"
|- valign="top"
! XQuery Type
! Java Type
|- valign="top"
| <code>xs:string</code>
| <code>String</code>, <code>char</code>, <code>Character</code>
|- valign="top"
| <code>xs:boolean</code>
| <code>boolean</code>, <code>Boolean</code>
|- valign="top"
| <code>xs:byte</code>
| <code>byte</code>, <code>Byte</code>
|- valign="top"
| <code>xs:short</code>
| <code>short</code>, <code>Short</code>
|- valign="top"
| <code>xs:int</code>
| <code>int</code>, <code>Integer</code>
|- valign="top"
| <code>xs:long</code>
| <code>long</code>, <code>Long</code>
|- valign="top"
| <code>xs:float</code>
| <code>float</code>, <code>Float</code>
|- valign="top"
| <code>xs:double</code>
| <code>double</code>, <code>Double</code>
|- valign="top"
| <code>xs:decimal</code>
| <code>java.math.BigDecimal</code>
|- valign="top"
| <code>xs:integer</code>
| <code>java.math.BigInteger</code>
|- valign="top"
| <code>xs:QName</code>
| <code>javax.xml.namespace.QName</code>
|- valign="top"
| <code>xs:anyURI</code>
| <code>java.net.URI</code>, <code>java.net.URL</code>
|- valign="top"
| ''empty sequence''
| <code>null</code>
|}
=Changelog=
; Version 8.4
* UpdatesUpdated: Rewriting rules
; Version 8.0
* Added: import of Java modules, context awareness
