Class JXPathContext

java.lang.Object
org.apache.commons.jxpath.JXPathContext
Direct Known Subclasses:
JXPathContextReferenceImpl
public abstract class JXPathContext extends Object
JXPathContext provides APIs for the traversal of graphs of JavaBeans using the XPath syntax. Using JXPathContext, you can read and write properties of JavaBeans, arrays, collections and maps. JXPathContext uses JavaBeans introspection to enumerate and access JavaBeans properties.

JXPathContext allows alternative implementations. This is why instead of allocating JXPathContext directly, you should call a static newContext method. This method will utilize the JXPathContextFactory API to locate a suitable implementation of JXPath. Bundled with JXPath comes a default implementation called Reference Implementation.

JXPath Interprets XPath Syntax on Java Object Graphs

JXPath uses an intuitive interpretation of the xpath syntax in the context of Java object graphs. Here are some examples:

Example 1: JavaBean Property Access

JXPath can be used to access properties of a JavaBean. 

public class Employee {
   public String getFirstName(){
      ...
   }
}

Employee emp = new Employee();
...

JXPathContext context = JXPathContext.newContext(emp);
String fName = (String)context.getValue("firstName");
In this example, we are using JXPath to access a property of the emp bean. In this simple case the invocation of JXPath is equivalent to invocation of getFirstName() on the bean.

Example 2: Nested Bean Property Access

JXPath can traverse object graphs: 

public class Employee {
   public Address getHomeAddress(){
      ...
   }
}
public class Address {
   public String getStreetNumber(){
      ...
   }
}

Employee emp = new Employee();
...

JXPathContext context = JXPathContext.newContext(emp);
String sNumber = (String)context.getValue("homeAddress/streetNumber");
In this case XPath is used to access a property of a nested bean.

A property identified by the xpath does not have to be a "leaf" property. For instance, we can extract the whole Address object in above example: 


   Address addr = (Address)context.getValue("homeAddress");

Example 3: Collection Subscripts

JXPath can extract elements from arrays and collections. 

public class Integers {
   public int[] getNumbers(){
      ...
   }
}

Integers ints = new Integers();
...

JXPathContext context = JXPathContext.newContext(ints);
Integer thirdInt = (Integer)context.getValue("numbers[3]");
A collection can be an arbitrary array or an instance of java.util. Collection.

Note: in XPath the first element of a collection has index 1, not 0.

Example 4: Map Element Access

JXPath supports maps. To get a value use its key. 

public class Employee {
   public Map getAddresses(){
      return addressMap;
   }

   public void addAddress(String key, Address address){
      addressMap.put(key, address);
   }
   ...
}

Employee emp = new Employee();
emp.addAddress("home", new Address(...));
emp.addAddress("office", new Address(...));
...

JXPathContext context = JXPathContext.newContext(emp);
String homeZipCode = (String)context.getValue("addresses/home/zipCode");
Often you will need to use the alternative syntax for accessing Map elements: 

String homeZipCode =
    (String) context.getValue("addresses[@name='home']/zipCode");
In this case, the key can be an expression, e.g. a variable.
Note: At this point JXPath only supports Maps that use strings for keys.
Note: JXPath supports the extended notion of Map: any object with dynamic properties can be handled by JXPath provided that its class is registered with the JXPathIntrospector.

Example 5: Retrieving Multiple Results

JXPath can retrieve multiple objects from a graph. Note that the method called in this case is not getValue, but iterate. 

public class Author {
   public Book[] getBooks(){
      ...
   }
}

Author auth = new Author();
...

JXPathContext context = JXPathContext.newContext(auth);
Iterator threeBooks = context.iterate("books[position() < 4]");
This returns a list of at most three books from the array of all books written by the author.

Example 6: Setting Properties

JXPath can be used to modify property values. 

public class Employee {
   public Address getAddress() {
      ...
   }

   public void setAddress(Address address) {
      ...
   }
}

Employee emp = new Employee();
Address addr = new Address();
...

JXPathContext context = JXPathContext.newContext(emp);
context.setValue("address", addr);
context.setValue("address/zipCode", "90190");

Example 7: Creating objects

JXPath can be used to create new objects. First, create a subclass of AbstractFactory and install it on the JXPathContext. Then call createPathAndSetValue() instead of "setValue". JXPathContext will invoke your AbstractFactory when it discovers that an intermediate node of the path is null. It will not override existing nodes. 

public class AddressFactory extends AbstractFactory {
   public boolean createObject(JXPathContext context,
              Pointer pointer, Object parent, String name, int index){
    if ((parent instanceof Employee) && name.equals("address"){
      ((Employee)parent).setAddress(new Address());
      return true;
    }
    return false;
  }
}

JXPathContext context = JXPathContext.newContext(emp);
context.setFactory(new AddressFactory());
context.createPathAndSetValue("address/zipCode", "90190");

Example 8: Using Variables

JXPath supports the notion of variables. The XPath syntax for accessing variables is "$varName". 

public class Author {
   public Book[] getBooks(){
      ...
   }
}

Author auth = new Author();
...

JXPathContext context = JXPathContext.newContext(auth);
context.getVariables().declareVariable("index", new Integer(2));

Book secondBook = (Book)context.getValue("books[$index]");
You can also set variables using JXPath: 

context.setValue("$index", new Integer(3));
Note: you can only change the value of an existing variable this way, you cannot define a new variable.

When a variable contains a JavaBean or a collection, you can traverse the bean or collection as well: 


...
context.getVariables().declareVariable("book", myBook);
String title = (String)context.getValue("$book/title);

Book array[] = new Book[]{...};

context.getVariables().declareVariable("books", array);

String title = (String)context.getValue("$books[2]/title);

Example 9: Using Nested Contexts

If you need to use the same set of variable while interpreting XPaths with different beans, it makes sense to put the variables in a separate context and specify that context as a parent context every time you allocate a new JXPathContext for a JavaBean. 

JXPathContext varContext = JXPathContext.newContext(null);
varContext.getVariables().declareVariable("title", "Java");

JXPathContext context = JXPathContext.newContext(varContext, auth);

Iterator javaBooks = context.iterate("books[title = $title]");

Using Custom Variable Pools

By default, JXPathContext creates a HashMap of variables. However, you can substitute a custom implementation of the Variables interface to make JXPath work with an alternative source of variables. For example, you can define implementations of Variables that cover a servlet context, HTTP request or any similar structure.

Example 10: Using Standard Extension Functions

Using the standard extension functions, you can call methods on objects, static methods on classes and create objects using any constructor. The class names should be fully qualified.

Here's how you can create new objects: 


Book book =
   (Book) context.getValue(
        "org.apache.commons.jxpath.example.Book.new ('John Updike')");
Here's how you can call static methods: 

  Book book =
   (Book) context.getValue(
      "org. apache.commons.jxpath.example.Book.getBestBook('John Updike')");
Here's how you can call regular methods: 

String firstName = (String)context.getValue("getAuthorsFirstName($book)");
As you can see, the target of the method is specified as the first parameter of the function.

Example 11: Using Custom Extension Functions

Collections of custom extension functions can be implemented as Functions objects or as Java classes, whose methods become extenstion functions.

Let's say the following class implements various formatting operations: 


public class Formats {
   public static String date(Date d, String pattern){
       return new SimpleDateFormat(pattern).format(d);
   }
   ...
}
We can register this class with a JXPathContext: 

context.setFunctions(new ClassFunctions(Formats.class, "format"));
...

context.getVariables().declareVariable("today", new Date());
String today = (String)context.getValue("format:date($today, 'MM/dd/yyyy')");
You can also register whole packages of Java classes using PackageFunctions.

Also, see FunctionLibrary, which is a class that allows you to register multiple sets of extension functions with the same JXPathContext.

Configuring JXPath

JXPath uses JavaBeans introspection to discover properties of JavaBeans. You can provide alternative property lists by supplying custom JXPathBeanInfo classes (see JXPathBeanInfo).

Notes

  • JXPath does not support DOM attributes for non-DOM objects. Even though XPaths like "para[@type='warning']" are legitimate, they will always produce empty results. The only attribute supported for JavaBeans is "name". The XPath "foo/bar" is equivalent to "foo[@name='bar']".
See XPath Tutorial by W3Schools
. Also see XML Path Language (XPath) Version 1.0

You will also find more information and examples in JXPath User's Guide
Version:
$Revision: 652845 $ $Date: 2008-05-02 12:46:46 -0500 (Fri, 02 May 2008) $
Author:
Dmitri Plotnikov

  • Field Details

    • contextFactory

      private static JXPathContextFactory contextFactory

    • compilationContext

      private static JXPathContext compilationContext

    • GENERIC_FUNCTIONS

      private static final PackageFunctions GENERIC_FUNCTIONS

    • parentContext

      protected JXPathContext parentContext
      parent context

    • contextBean

      protected Object contextBean
      context bean

    • vars

      protected Variables vars
      variables

    • functions

      protected Functions functions
      functions

    • factory

      protected AbstractFactory factory
      AbstractFactory

    • idManager

      protected IdentityManager idManager
      IdentityManager

    • keyManager

      protected KeyManager keyManager
      KeyManager

    • decimalFormats

      protected HashMap decimalFormats
      decimal format map

    • locale

      private Locale locale

    • lenientSet

      private boolean lenientSet

    • lenient

      private boolean lenient

  • Constructor Details

    • JXPathContext

      protected JXPathContext(JXPathContext parentContext, Object contextBean)
      This constructor should remain protected - it is to be overridden by subclasses, but never explicitly invoked by clients.
      Parameters:
      parentContext - parent context
      contextBean - Object

  • Method Details

    • newContext

      public static JXPathContext newContext(Object contextBean)
      Creates a new JXPathContext with the specified object as the root node.
      Parameters:
      contextBean - Object
      Returns:
      JXPathContext

    • newContext

      public static JXPathContext newContext(JXPathContext parentContext, Object contextBean)
      Creates a new JXPathContext with the specified bean as the root node and the specified parent context. Variables defined in a parent context can be referenced in XPaths passed to the child context.
      Parameters:
      parentContext - parent context
      contextBean - Object
      Returns:
      JXPathContext

    • getContextFactory

      private static JXPathContextFactory getContextFactory()
      Acquires a context factory and caches it.
      Returns:
      JXPathContextFactory

    • getParentContext

      public JXPathContext getParentContext()
      Returns the parent context of this context or null.
      Returns:
      JXPathContext

    • getContextBean

      public Object getContextBean()
      Returns the JavaBean associated with this context.
      Returns:
      Object

    • getContextPointer

      public abstract Pointer getContextPointer()
      Returns a Pointer for the context bean.
      Returns:
      Pointer

    • getRelativeContext

      public abstract JXPathContext getRelativeContext(Pointer pointer)
      Returns a JXPathContext that is relative to the current JXPathContext. The supplied pointer becomes the context pointer of the new context. The relative context inherits variables, extension functions, locale etc from the parent context.
      Parameters:
      pointer - Pointer
      Returns:
      JXPathContext

    • setVariables

      public void setVariables(Variables vars)
      Installs a custom implementation of the Variables interface.
      Parameters:
      vars - Variables

    • getVariables

      public Variables getVariables()
      Returns the variable pool associated with the context. If no such pool was specified with the setVariables(Variables) method, returns the default implementation of Variables, BasicVariables.
      Returns:
      Variables

    • setFunctions

      public void setFunctions(Functions functions)
      Install a library of extension functions.
      Parameters:
      functions - Functions
      See Also:

    • getFunctions

      public Functions getFunctions()
      Returns the set of functions installed on the context.
      Returns:
      Functions

    • setFactory

      public void setFactory(AbstractFactory factory)
      Install an abstract factory that should be used by the createPath() and createPathAndSetValue() methods.
      Parameters:
      factory - AbstractFactory

    • getFactory

      public AbstractFactory getFactory()
      Returns the AbstractFactory installed on this context. If none has been installed and this context has a parent context, returns the parent's factory. Otherwise returns null.
      Returns:
      AbstractFactory

    • setLocale

      public void setLocale(Locale locale)
      Set the locale for this context. The value of the "lang" attribute as well as the the lang() function will be affected by the locale. By default, JXPath uses Locale.getDefault()
      Parameters:
      locale - Locale

    • getLocale

      public Locale getLocale()
      Returns the locale set with setLocale. If none was set and the context has a parent, returns the parent's locale. Otherwise, returns Locale.getDefault().
      Returns:
      Locale

    • setDecimalFormatSymbols

      public void setDecimalFormatSymbols(String name, DecimalFormatSymbols symbols)
      Sets DecimalFormatSymbols for a given name. The DecimalFormatSymbols can be referenced as the third, optional argument in the invocation of format-number (number,format,decimal-format-name) function. By default, JXPath uses the symbols for the current locale.
      Parameters:
      name - the format name or null for default format.
      symbols - DecimalFormatSymbols

    • getDecimalFormatSymbols

      public DecimalFormatSymbols getDecimalFormatSymbols(String name)
      Get the named DecimalFormatSymbols.
      Parameters:
      name - key
      Returns:
      DecimalFormatSymbols
      See Also:

    • setLenient

      public void setLenient(boolean lenient)
      If the context is in the lenient mode, then getValue() returns null for inexistent paths. Otherwise, a path that does not map to an existing property will throw an exception. Note that if the property exists, but its value is null, the exception is not thrown.

      By default, lenient = false

      Parameters:
      lenient - flag

    • isLenient

      public boolean isLenient()
      Learn whether this JXPathContext is lenient.
      Returns:
      boolean
      See Also:

    • compile

      public static CompiledExpression compile(String xpath)
      Compiles the supplied XPath and returns an internal representation of the path that can then be evaluated. Use CompiledExpressions when you need to evaluate the same expression multiple times and there is a convenient place to cache CompiledExpression between invocations.
      Parameters:
      xpath - to compile
      Returns:
      CompiledExpression

    • compilePath

      protected abstract CompiledExpression compilePath(String xpath)
      Overridden by each concrete implementation of JXPathContext to perform compilation. Is called by compile().
      Parameters:
      xpath - to compile
      Returns:
      CompiledExpression

    • selectSingleNode

      public Object selectSingleNode(String xpath)
      Finds the first object that matches the specified XPath. It is equivalent to getPointer(xpath).getNode(). Note that this method produces the same result as getValue() on object models like JavaBeans, but a different result for DOM/JDOM etc., because it returns the Node itself, rather than its textual contents.
      Parameters:
      xpath - the xpath to be evaluated
      Returns:
      the found object

    • selectNodes

      public List selectNodes(String xpath)
      Finds all nodes that match the specified XPath.
      Parameters:
      xpath - the xpath to be evaluated
      Returns:
      a list of found objects

    • getValue

      public abstract Object getValue(String xpath)
      Evaluates the xpath and returns the resulting object. Primitive types are wrapped into objects.
      Parameters:
      xpath - to evaluate
      Returns:
      Object found

    • getValue

      public abstract Object getValue(String xpath, Class requiredType)
      Evaluates the xpath, converts the result to the specified class and returns the resulting object.
      Parameters:
      xpath - to evaluate
      requiredType - required type
      Returns:
      Object found

    • setValue

      public abstract void setValue(String xpath, Object value)
      Modifies the value of the property described by the supplied xpath. Will throw an exception if one of the following conditions occurs:
      • The xpath does not in fact describe an existing property
      • The property is not writable (no public, non-static set method)
      Parameters:
      xpath - indicating position
      value - to set

    • createPath

      public abstract Pointer createPath(String xpath)
      Creates missing elements of the path by invoking an AbstractFactory, which should first be installed on the context by calling setFactory(AbstractFactory).

      Will throw an exception if the AbstractFactory fails to create an instance for a path element.

      Parameters:
      xpath - indicating destination to create
      Returns:
      pointer to new location

    • createPathAndSetValue

      public abstract Pointer createPathAndSetValue(String xpath, Object value)
      The same as setValue, except it creates intermediate elements of the path by invoking an AbstractFactory, which should first be installed on the context by calling setFactory(AbstractFactory).

      Will throw an exception if one of the following conditions occurs:

      • Elements of the xpath aleady exist, but the path does not in fact describe an existing property
      • The AbstractFactory fails to create an instance for an intermediate element.
      • The property is not writable (no public, non-static set method)
      Parameters:
      xpath - indicating position to create
      value - to set
      Returns:
      pointer to new location

    • removePath

      public abstract void removePath(String xpath)
      Removes the element of the object graph described by the xpath.
      Parameters:
      xpath - indicating position to remove

    • removeAll

      public abstract void removeAll(String xpath)
      Removes all elements of the object graph described by the xpath.
      Parameters:
      xpath - indicating positions to remove

    • iterate

      public abstract Iterator iterate(String xpath)
      Traverses the xpath and returns an Iterator of all results found for the path. If the xpath matches no properties in the graph, the Iterator will be empty, but not null.
      Parameters:
      xpath - to iterate
      Returns:
      Iterator

    • getPointer

      public abstract Pointer getPointer(String xpath)
      Traverses the xpath and returns a Pointer. A Pointer provides easy access to a property. If the xpath matches no properties in the graph, the pointer will be null.
      Parameters:
      xpath - desired
      Returns:
      Pointer

    • iteratePointers

      public abstract Iterator iteratePointers(String xpath)
      Traverses the xpath and returns an Iterator of Pointers. A Pointer provides easy access to a property. If the xpath matches no properties in the graph, the Iterator be empty, but not null.
      Parameters:
      xpath - to iterate
      Returns:
      Iterator

    • setIdentityManager

      public void setIdentityManager(IdentityManager idManager)
      Install an identity manager that will be used by the context to look up a node by its ID.
      Parameters:
      idManager - IdentityManager to set

    • getIdentityManager

      public IdentityManager getIdentityManager()
      Returns this context's identity manager. If none has been installed, returns the identity manager of the parent context.
      Returns:
      IdentityManager

    • getPointerByID

      public Pointer getPointerByID(String id)
      Locates a Node by its ID.
      Parameters:
      id - is the ID of the sought node.
      Returns:
      Pointer

    • setKeyManager

      public void setKeyManager(KeyManager keyManager)
      Install a key manager that will be used by the context to look up a node by a key value.
      Parameters:
      keyManager - KeyManager

    • getKeyManager

      public KeyManager getKeyManager()
      Returns this context's key manager. If none has been installed, returns the key manager of the parent context.
      Returns:
      KeyManager

    • getPointerByKey

      public Pointer getPointerByKey(String key, String value)
      Locates a Node by a key value.
      Parameters:
      key - string
      value - string
      Returns:
      Pointer found

    • getNodeSetByKey

      public NodeSet getNodeSetByKey(String key, Object value)
      Locates a NodeSet by key/value.
      Parameters:
      key - string
      value - object
      Returns:
      NodeSet found

    • registerNamespace

      public void registerNamespace(String prefix, String namespaceURI)
      Registers a namespace prefix.
      Parameters:
      prefix - A namespace prefix
      namespaceURI - A URI for that prefix

    • getNamespaceURI

      public String getNamespaceURI(String prefix)
      Given a prefix, returns a registered namespace URI. If the requested prefix was not defined explicitly using the registerNamespace method, JXPathContext will then check the context node to see if the prefix is defined there. See setNamespaceContextPointer.
      Parameters:
      prefix - The namespace prefix to look up
      Returns:
      namespace URI or null if the prefix is undefined.

    • getPrefix

      public String getPrefix(String namespaceURI)
      Get the prefix associated with the specifed namespace URI.
      Parameters:
      namespaceURI - the ns URI to check.
      Returns:
      String prefix
      Since:
      JXPath 1.3

    • setNamespaceContextPointer

      public void setNamespaceContextPointer(Pointer namespaceContextPointer)
      Namespace prefixes can be defined implicitly by specifying a pointer to a context where the namespaces are defined. By default, NamespaceContextPointer is the same as the Context Pointer, see getContextPointer()
      Parameters:
      namespaceContextPointer - The pointer to the context where prefixes used in XPath expressions should be resolved.

    • getNamespaceContextPointer

      public Pointer getNamespaceContextPointer()
      Returns the namespace context pointer set with setNamespaceContextPointer() or, if none has been specified, the context pointer otherwise.
      Returns:
      The namespace context pointer.