Difference between revisions of "Java Bindings"
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The new [[XQuery 3.0#Expanded QNames|Expanded QName]] notation of XQuery 3.0 | The new [[XQuery 3.0#Expanded QNames|Expanded QName]] notation of XQuery 3.0 | ||
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<pre class="brush:xquery"> | <pre class="brush:xquery"> |
Revision as of 15:45, 11 November 2013
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.
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.
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:
- 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. As a consequence, the addressed class must provide a constructor with no arguments.
Context-Awareness
Template:Mark: 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:
- 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 users with less privileges. - Java code is treated as non-deterministic, as its behavior cannot be predicted by 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 = '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.
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.
Changelog
- Version 7.8
- Added: Java locking annotations
- Updated:
context
variable has been split intoqueryContext
andstaticContext
.
- Version 7.2.1
- Added: import of Java modules, context awareness