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 (since Version 7.2.1) 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 be of the form java:fully.qualified.ClassName.

Namespace Declarations

The following example uses Java’s Math class and returns the cosine of an angle. The cos() method can be directly called, as it is a static method:

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

The next example writes 256 bytes to the file output.txt. First, a new FileWriter instance is created: by calling the new() function, the class constructor is invoked. Instance methods are called by passing on the resulting Java object as first argument:

declare namespace fw = "java: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)
)

In general, it is recommended to use XQuery expressions and functions whenever possible, as Java code cannot be pre-compiled, and will often be evaluated slower than optimized XQuery code. Next, Java code can only be executed with admin permissions.

Module Imports

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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:java.util.HashSet";
let $loop :=
  for $i in 1 to 10000
  return set:add($i)
return set:size()

Advantages of this approach are:

  • imported code can be executed faster than instances created at runtime via new().
  • the work on class instances ensures that queries run in parallel will not cause any concurrency issues (provided that the class contains no static variables or functions).

A drawback is that no arguments can be passed on to the class constructor. This is also why the class must provide a constructor without no arguments.

Context-Awareness

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Context-aware classes represent a powerful interface for writing Java modules that are more closely coupled with the BaseX core library. If an instantiated class inherits the abstract QueryModule class of BaseX, it will get access to the context variable, which is an instance of the QueryContext class. It provides access to all static and dynamic properties of the current query. Additionally, the default properties of functions can be changed via annotations:

  • Java functions can only be executed by users with ADMIN permissions. You may annotate a function with Requires(Permission) to also make it accessible to other users with less permissions.
  • Java code is treated as "non-deterministic", as its behavior cannot be predicted from the XQuery processor. You may annotate a function as Deterministic if you know that it will have no side-effects and will always yield the same result.
  • Java code is treated as "context-independent". If a function accesses the query context, it should be annotated as ContextDependent
  • Java code is treated as "focus-independent". If a function accesses the current context item, position or size, it should be annotated as FocusDependent

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 = 'java:org.basex.examples.query.ContextModule';

<context>{
  context:function-namespace()
}</context>,
<to-int>{
  try { context:to-int('abc') }
  catch * { 'Error in line', $err:line-number }
}</to-int>

The imported Java class is shown below:

package org.basex.examples.query;

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

/**
 * This example is inherited from the {@link QueryModule} class.
 */
public class ContextModule extends QueryModule {
  /**
   * Returns the default function namespace.
   * @return default function namespace
   */
  @Requires(Permissions.NONE)
  @Deterministic
  @ContextDependent
  public Str functionNS() {
    return Str.get(context.sc.nsFunc);
  }

  /**
   * Converts the specified string to an integer.
   * @param value string representation
   * @return integer
   * @throws QueryException query exception
   */
  @Requires(Permissions.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:

<context>http://www.w3.org/2005/xpath-functions</context>
<to-int>Error in line 6</to-int>

Changelog

Version 7.2.1

  • Added: import of Java modules, context awareness