| 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 AbstractFactory and install it on the JXPathContext. Then
call
JXPathContext.createPath 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 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 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 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
author:
Dmitri Plotnikov
version:
$Revision: 1.25 $ $Date: 2004/06/29 21:15:46 $ | 