Java Bindings

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This article is part of the XQuery Portal. It demonstrates two ways to invoke Java code from XQuery and an extension to make Java code aware of the current context.

The Java Binding feature is an extensibility mechanism which enables developers to directly access Java variables and execute code from XQuery. Java classes are identified by namespaces. The namespace URI must simply contain the fully qualified class name. The URI can optionally be prefixed with the string java: to enforce that the addressed code is written in Java.

If the addressed Java code is not found in the classpath, it first needs to be installed in the Repository.

Contents

[edit] Namespace Declarations

Java classes can be declared via namespaces. The namespace can then be used to call static functions contained in that class. Variables are represented as function with 0 parameters.

The following example uses Java’s Math class to return the cosine of an angle by calling the static method cos(), and the value of π by addressing the static variable via PI():

declare namespace math = "java:java.lang.Math";
math:cos(xs:double(0)), math:PI()

The new Expanded QName notation of XQuery 3.0 allows you to directly specify a namespace URI instead of the prefix:

Q{java:java.lang.Math}cos(xs:double(0))

The constructor of a class can be invoked by calling the virtual function 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 output.txt. First, a new FileWriter instance is created, and its write() function is called in the next step. The java: prefix is omitted in the URI:

declare namespace fw = "java.io.FileWriter";
let $file := fw:new('output.txt')
return (
  for $i in 0 to 255
  return fw:write($file, xs:int($i)),
  fw:close($file)
)

Function names with dashes will be rewritten to Java’s camel case notation:

XQuery: get-contents($x as xs:string) 
Java  : getContents(String x)

Strings with invalid XML characters will be rejected by default. The validity check can be disabled by setting the CHECKSTRINGS option to false. The following query writes a file with a single 00-byte, which will then be successfully read via Java functions:

declare namespace br = 'java.io.BufferedReader';
declare namespace fr = 'java.io.FileReader';

declare option db:checkstrings 'false';

file:write-binary('00.bin', xs:hexBinary('00')),
br:new(fr:new('00.bin')) ! (br:readLine(.), br:close(.))

Note that Java code cannot be pre-compiled, and will often be evaluated slower than optimized XQuery code.

[edit] Module Imports

Java code can also be integrated by importing classes as modules. A new instance of the addressed class is created, which can then be accessed in the query body.

An example (the boolean values returned by set:add() are ignored):

import module namespace set = "java.util.HashSet";
let $loop := (
  set:add("check"),
  set:add("what"),
  set:add("happens")
)
return set:size()

Advantages of this approach are:

A drawback is that no arguments can be passed on to the class constructor. As a consequence, the addressed class must provide a constructor with no arguments.

[edit] Context-Awareness

Updated with Version 7.8: context variable has been split into queryContext and staticContext.

Java classes can be coupled more closely to the BaseX core library. If a class inherits the abstract QueryModule class, the two variables queryContext and 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:

The following XQuery code invokes two Java methods. The first Java function retrieves information from the static query context, and the second one throws a query exception:

import module namespace context = 'org.basex.examples.query.ContextModule';

element user {
  context:user()
},
element to-int {
  try { context:to-int('abc') }
  catch * { 'Error in line', $err:line-number }
}

The imported Java class is shown below:

package org.basex.examples.query;

import org.basex.query.*;
import org.basex.query.value.item.*;
import org.basex.util.*;

/**
 * This example is inherited from the {@link QueryModule} class.
 */
public class ContextModule extends QueryModule {
  /**
   * Returns the name of the logged in user.
   * @return user
   */
  @Requires(Permission.NONE)
  @Deterministic
  @ContextDependent
  public String user() {
    return queryContext.context.user.name;
  }

  /**
   * Converts the specified string to an integer.
   * @param value string representation
   * @return integer
   * @throws QueryException query exception
   */
  @Requires(Permission.NONE)
  @Deterministic
  public int toInt(final String value) throws QueryException {
    try {
      return Integer.parseInt(value);
    } catch(NumberFormatException ex) {
      throw new QueryException(ex.getMessage());
    }
  }
}

The result will look as follows:

<user>admin</admin>
<to-int>Error in line 6</to-int>

Please visit the XQuery 3.0 specification if you want to get more insight into function properties.

[edit] Locking

Introduced with Version 7.8:

By default, a Java function will be executed in parallel with other code. However, if a Java function performs sensitive write operations, it is advisable to explicitly lock the code. This can be realized via locking annotations:

  @Lock(write = { "HEAVYIO" })
  public void write() {
    // ...
  }

  @Lock(read = { "HEAVYIO" })
  public void read() {
    // ...
  }

If an XQuery expression is run which calls the Java write() function, every other query that calls write() or read() needs to wait for the query to be finished. If a query calls the read() function, only those queries are queued that call write(), because this function is only annotated with a read lock. More details on parallel query execution can be found in the article on Transaction Management.

[edit] Changelog

Version 7.8
Version 7.2.1
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