Difference between revisions of "Java Bindings"

From BaseX Documentation
Jump to navigation Jump to search
(47 intermediate revisions by 2 users not shown)
Line 1: Line 1:
This article is part of the [[XQuery|XQuery Portal]].
+
This article is part of the [[XQuery|XQuery Portal]]. It demonstrates different ways to invoke Java code from XQuery, and it presents extensions to access the current query context from Java.
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
 
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
+
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]].
namespaces. The namespace URI must simply contain the fully qualified class name.
 
The URI can optionally be prefixed with the string {{Code|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
+
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.
installed in the [[Repository]].
+
 
 +
=Identification=
 +
 
 +
==Classes==
 +
 
 +
A Java class is identified by a namespace URI. The original URI is rewritten as follows:
 +
 
 +
# The [[Repository#URI_Rewriting|URI Rewriting]] steps are applied to the URI.
 +
# Slashes in the resulting URI are replaced with dots.
 +
# 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>
 +
* <code>java:java.lang.String</code> → <code>java.lang.String</code>
 +
 
 +
==Functions and Variables==
 +
 
 +
Java functions and variables can be referenced and evaluated by the existing XQuery function syntax:
 +
 
 +
* 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 &amp;#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>
 +
|- 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)]</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=
 
=Namespace Declarations=
  
Java classes can be declared via namespaces. The namespace can then be used to
+
In the following example, Java’s {{Code|Math}} class is referenced. When executed, the query returns the cosine of an angle by calling the static method {{Code|cos()}}, and the value of π by addressing the static variable via {{Code|PI()}}:
call static functions contained in that class. Variables are represented as
 
function with 0 parameters.
 
 
 
The following example uses Java’s {{Code|Math}} class to return the cosine of an angle
 
by calling the static method {{Code|cos()}}, and the value of π by addressing the static
 
variable via {{Code|PI()}}:
 
  
 
<pre class="brush:xquery">
 
<pre class="brush:xquery">
Line 27: Line 61:
 
</pre>
 
</pre>
  
The new [[XQuery 3.0#Expanded QNames|Expanded QName]] notation of XQuery 3.0
+
With the [[XQuery 3.0#Expanded QNames|Expanded QName]] notation of XQuery 3.0,
can be applied as well to directly specify a namespace URI instead of the prefix:
+
the namespace can directly be embedded in the function call:
  
 
<pre class="brush:xquery">
 
<pre class="brush:xquery">
Line 34: Line 68:
 
</pre>
 
</pre>
  
The constructor of a class can be invoked by calling the virtual
+
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:
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. The {{Code|java:}} prefix is omitted in
 
the URI:
 
  
 
<pre class="brush:xquery">declare namespace fw = "java.io.FileWriter";
 
<pre class="brush:xquery">declare namespace fw = "java.io.FileWriter";
Line 52: Line 79:
 
</pre>
 
</pre>
  
Function names with dashes will be rewritten to Java’s camel case notation:
+
If the result of a Java call contains invalid XML characters, it will be rejected. The validity check can be disabled by setting the [[Options#CHECKSTRINGS|CHECKSTRINGS]] option to false. The following query writes a file with a single 00-byte, which will then be successfully read via Java functions:
 
 
<pre class="brush:xquery">
 
XQuery: get-contents($x as xs:string)
 
Java : getContents(String x)
 
</pre>
 
 
 
Strings with invalid XML characters will be rejected by default. The validity check can be disabled by setting the [[Options#CHECKSTRINGS|CHECKSTRINGS]] option to false. The following query writes a file with a single 00-byte, which will then be successfully read via Java functions:
 
  
 
<pre class="brush:xquery">
 
<pre class="brush:xquery">
Line 71: Line 91:
 
</pre>
 
</pre>
  
Note that Java code cannot be pre-compiled, and will often be evaluated slower than optimized
+
Note that Java code cannot be pre-compiled, and will as such be evaluated slower than optimized XQuery code.
XQuery code.
 
  
 
=Module Imports=
 
=Module Imports=
  
Java code can also be integrated by ''importing'' classes as modules.
+
Java code can also be accessed by ''importing'' classes as modules. A new instance of the addressed class will be created, which can then be referenced in the query body.
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 {{Code|set:add()}} are ignored):
+
The following (side-effecting) example returns the number of distinct values added to a hash set. The boolean values returned by {{Code|set:add()}} will be swallowed:
  
 
<pre class="brush:xquery">
 
<pre class="brush:xquery">
import module namespace set = "java.util.HashSet";
+
import module namespace set = "java:java.util.HashSet";
let $loop :=
+
prof:void(
   for $i in 1 to 10000
+
   for $s in ("one", "two", "one")
   return set:add($i)
+
   return set:add($s)
return set:size()
+
),
 +
set:size()
 
</pre>
 
</pre>
  
Advantages of this approach are:
+
The advantages of this approach is the execution of imported classes is 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 fails if the addressed class has no default constructor, but at least one constructor with arguments.
* imported code can be executed faster than instances created at runtime via {{Code|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=
 
=Context-Awareness=
  
{{Mark|Updated with Version 7.8}}: {{Code|context}} variable has been split into {{Code|queryContext}} and {{Code|staticContext}}.
+
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:
Java classes can be coupled more closely to the BaseX core library.
 
If a class inherits the abstract [https://github.com/BaseXdb/basex/blob/master/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:
 
  
 
* Java functions can only be executed by users with [[User_Management|Admin permissions]]. You may annotate a function with {{Code|@Requires(<Permission>)}} to also make it accessible to users with less privileges.
 
* Java functions can only be executed by users with [[User_Management|Admin permissions]]. You may annotate a function with {{Code|@Requires(<Permission>)}} to also make it accessible to users with less privileges.
Line 107: Line 119:
 
* Java code is treated as ''context-independent''. If a function accesses the query context, it should be annotated as {{Code|@ContextDependent}}
 
* Java code is treated as ''context-independent''. If a function accesses the query context, it should be annotated as {{Code|@ContextDependent}}
 
* Java code is treated as ''focus-independent''. If a function accesses the current context item, position or size, it should be annotated as {{Code|@FocusDependent}}
 
* Java code is treated as ''focus-independent''. If a function accesses the current context item, position or size, it should be annotated as {{Code|@FocusDependent}}
 +
 +
The [https://github.com/BaseXdb/basex/blob/master/basex-core/src/main/java/org/basex/query/QueryResource.java QueryResource] interface can be implemented to enforce finalizing operations, such as the closing of opened connections or resources in a module. Its {{Code|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:
 
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:
Line 132: Line 146:
  
 
/**
 
/**
  * This example is inherited from the {@link QueryModule} class.
+
  * This example inherits the {@link QueryModule} class and
 +
* implements the QueryResource interface.
 
  */
 
  */
public class ContextModule extends QueryModule {
+
public class ContextModule extends QueryModule implements QueryResource {
 
   /**
 
   /**
 
   * Returns the name of the logged in user.
 
   * Returns the name of the logged in user.
Line 160: Line 175:
 
       throw new QueryException(ex.getMessage());
 
       throw new QueryException(ex.getMessage());
 
     }
 
     }
 +
  }
 +
 +
  @Override
 +
  public void close() {
 +
    // see description above
 
   }
 
   }
 
}
 
}
Line 175: Line 195:
  
 
=Locking=
 
=Locking=
 
{{Mark|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:
 
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:
Line 193: Line 211:
  
 
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]].
 
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=
 
=Changelog=
 +
 +
; Version 8.4
 +
 +
* Updated: Rewriting rules
 +
 +
; Version 8.0
 +
 +
* Added: {{Code|QueryResource}} interface, called after a query has been fully evaluated.
  
 
; Version 7.8
 
; Version 7.8
Line 204: Line 279:
  
 
* Added: import of Java modules, context awareness
 
* Added: import of Java modules, context awareness
 
[[Category:XQuery]]
 
[[Category:API]]
 

Revision as of 17:42, 17 October 2017

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.

Identification

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:

  • http://basex.org/modules/meta-dataorg.basex.modules.MetaData
  • java:java.lang.Stringjava.lang.String

Functions and Variables

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

  • 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 · (&#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.
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.

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.

Module Imports

Java code can also be accessed by importing classes as modules. A new instance of the addressed class will be created, which can then be referenced 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:java.util.HashSet";
prof:void(
  for $s in ("one", "two", "one")
  return set:add($s)
),
set:size()

The advantages of this approach is the execution of imported classes is more efficient than the execution of instances that are created at runtime via new(). A drawback is that no arguments can be passed on to the class constructor. As a consequence, the import fails if the addressed class has no default constructor, but at least one constructor with arguments.

Context-Awareness

Java classes can be coupled even 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 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.

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.

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

Changelog

Version 8.4
  • Updated: Rewriting rules
Version 8.0
  • Added: QueryResource interface, called after a query has been fully evaluated.
Version 7.8
  • Added: Java locking annotations
  • Updated: context variable has been split into queryContext and staticContext.
Version 7.2.1
  • Added: import of Java modules, context awareness