Java Bindings

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This article is part of the XQuery Portal. It demonstrates different ways to invoke Java code from XQuery, and it presents extensions to access the current query context from Java.

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.

Contents

[edit] Identification

[edit] Classes

A Java class is identified by a namespace URI. The original URI is rewritten as follows:

  1. The URI Rewriting steps are applied to the URI.
  2. Slashes in the resulting URI are replaced with dots.
  3. The last path segment of the URI is capitalized and rewritten to camel case.

The normalization steps are skipped if the URI is prefixed with java:. See the following examples:

[edit] Functions and Variables

Java functions and variables can be referenced and evaluated by the existing XQuery function syntax:

Type XQuery Java
Variable Q{java.lang.Integer}MIN_VALUE Integer.MIN_VALUE
Function Q{java.lang.Object}hash-code() object.hashCode()
Function with types Q{java.lang.String}split·java.lang.String·int(';', 3) string.split(";", 3)

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).

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 int value, you will need to convert your XQuery integers to xs:int.

[edit] Namespace Declarations

In the following example, Java’s Math class is referenced. When executed, the query returns 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()

With the Expanded QName notation of XQuery 3.0, the namespace can directly be embedded in the function call:

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:

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)
)

If the result of a Java call contains invalid XML characters, it will be rejected. 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 as such 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.

The following, side-effecting example returns the number of distinct values added to a hash set (the boolean values returned by set:add() will be swallowed):

import module namespace set = "java.util.HashSet";
prof:void(
  for $s in ("one", "two", "one")
  return set:add($s)
),
set:size()

The advantages of this approach is that imported code is executed faster than instances created at runtime via new(). A drawback is that no arguments can be passed on to the class constructor. As a consequence, the import will fail if the class has at least one constructor with arguments and no default constructor.

[edit] Context-Awareness

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 QueryResource interface can be implemented to enforce finalizing operations, such as the closing of opened connections or resources in a module. Its close() method will be called after a query has been fully evaluated.

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 inherits the {@link QueryModule} class and
 * implements the QueryResource interface.
 */
public class ContextModule extends QueryModule implements QueryResource {
  /**
   * 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());
    }
  }

  @Override
  public void close() {
    // see description above
  }
}

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

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] Data Types

The following table lists the mappings of XQuery and Java types:

XQuery Type Java Type
xs:string String, char, Character
xs:boolean boolean, Boolean
xs:byte byte, Byte
xs:short short, Short
xs:int int, Integer
xs:long long, Long
xs:float float, Float
xs:double double, Double
xs:decimal java.math.BigDecimal
xs:integer java.math.BigInteger
xs:QName javax.xml.namespace.QName
xs:anyURI java.net.URI, java.net.URL
empty sequence null

[edit] Changelog

Version 8.4
Version 8.0
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Version 7.2.1
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