The Bean Introspection Utilities component of the Jakarta Commons subproject offers low-level utility classes that assist in getting and setting property values on Java classes that follow the naming design patterns outlined in the JavaBeans Specification, as well as mechanisms for dynamically defining and accessing bean properties.

# Table of Contents

1. Overview
   - Background
   - External Dependencies
2. Standard JavaBeans
   - Background
   - Basic Property Access
   - Nested Property Access
3. Dynamic Beans (DynaBeans)
   - Background
   - BasicDynaBean and BasicDynaClass
   - ResultSetDynaClass (Wraps ResultSet in DynaBeans)
   - RowSetDynaClass (Disconnected ResultSet as DynaBeans)
   - WrapDynaBean and WrapDynaClass
   - Lazy DynaBeans
4. Data Type Conversions
   - Background
   - BeanUtils and ConvertUtils Conversions
   - Defining Your Own Converters
   - Locale Aware Conversions
5. Utility Objects And Static Utility Classes
   - Background
6. Collections
   - Comparing Beans
   - Operating On Collections Of Beans
   - Querying Or Filtering Collections Of Beans
   - Transforming Collections Of Beans
7. Frequently Asked Questions
   - Why Can't BeanUtils Find My Method?
   - How Do I Set The BeanComparator Order To Be Ascending/Descending?
8. Packaging

# Overview

### Background

The JavaBeans name comes from a Java API specification for a component architecture for the Java language. Writing Java classes that conform to the JavaBeans design patterns makes it easier for Java developers to understand the functionality provided by your class, as well as allowing JavaBeans-aware tools to use Java's introspection capabilities to learn about the properties and operations provided by your class, and present them in a visually appealing manner in development tools.

The JavaBeans Specification describes the complete set of characteristics that makes an arbitrary Java class a JavaBean or not -- and you should consider reading this document to be an important part of developing your Java programming skills. However, the required characteristics of JavaBeans that are important for most development scenarios are listed here:

- The class must be public, and provide a public constructor that accepts no arguments. This allows tools and applications to dynamically create new instances of your bean, without necessarily knowing what Java class name will be used ahead of time, like this:

              String className = ...;
              Class beanClass = Class.forName(className);
              Object beanInstance = beanClass.newInstance();

- As a necessary consequence of having a no-arguments constructor, configuration of your bean's behavior must be accomplished separately from its instantiation. This is typically done by defining a set of properties of your bean, which can be used to modify its behavior or the data that the bean represents. The normal convention for property names is that they start with a lower case letter, and be comprised only of characters that are legal in a Java identifier.
- Typically, each bean property will have a public getter and setter method that are used to retrieve or define the property's value, respectively. The JavaBeans Specification defines a design pattern for these names, using `get` or `set` as the prefix for the property name with it's first character capitalized. Thus, you a JavaBean representing an employee might have (among others) properties named `firstName`, `lastName`, and `hireDate`, with method signatures like this:

              public class Employee {
                  public Employee();   // Zero-arguments constructor
                  public String getFirstName();
                  public void setFirstName(String firstName);
                  public String getLastName();
                  public void setLastName(String lastName);
                  public Date getHireDate();
                  public void setHireDate(Date hireDate);
                  public boolean isManager();
                  public void setManager(boolean manager);
                  public String getFullName();
              }

- As you can see from the above example, there is a special variant allowed for boolean properties -- you can name the getter method with a `is` prefix instead of a `get` prefix if that makes for a more understandable method name.
- If you have both a getter and a setter method for a property, the data type returned by the getter must match the data type accepted by the setter. In addition, it is contrary to the JavaBeans specification to have more than one setter with the same name, but different property types.
- It is not required that you provide a getter and a setter for every property. In the example above, the `fullName` property is read-only, because there is no setter method. It is also possible, but less common, to provide write-only properties.
- It is also possible to create a JavaBean where the getter and setter methods do not match the naming pattern described above. The standard JavaBeans support classes in the Java language, as well as all classes in the BeanUtils package, allow you to describe the actual property method names in a `BeanInfo` class associated with your bean class. See the JavaBeans Specification for full details.
- The JavaBeans Specification also describes many additional design patterns for event listeners, wiring JavaBeans together into component hierarchies, and other useful features that are beyond the scope of the BeanUtils package.

Using standard Java coding techniques, it is very easy to deal with JavaBeans if you know ahead of time which bean classes you will be using, and which properties you are interested in:

            Employee employee = ...;
            System.out.println("Hello " + employee.getFirstName() + "!");

### External Dependencies

The commons-beanutils package requires that the following additional packages be available in the application's class path at runtime:

- Collections Package (Jakarta Commons), version 1.0 or later
- Logging Package (Jakarta Commons), version 1.0 or later

# Standard JavaBeans

### Background

As described above, the standard facilities of the Java programming language make it easy and natural to access the property values of your beans using calls to the appropriate getter methods. But what happens in more sophisticated environments where you do not necessarily know ahead of time which bean class you are going to be using, or which property you want to retrieve or modify? The Java language provides classes like `java.beans.Introspector`, which can examine a Java class at runtime and identify for you the names of the property getter and setter methods, plus the Reflection capabilities to dynamically call such a method. However, these APIs can be difficult to use, and expose the application developer to many unnecessary details of the underlying structure of Java classes. The APIs in the BeanUtils package are intended to simplify getting and setting bean properties dynamically, where the objects you are accessing -- and the names of the properties you care about -- are determined at runtime in your application, rather than as you are writing and compiling your application's classes.

This is the set of needs that are satisfied by the static methods of the PropertyUtils class, which are described further in this section. First, however, some further definitions will prove to be useful:

The general set of possible property types supported by a JavaBean can be broken into three categories -- some of which are supported by the standard JavaBeans specification, and some of which are uniquely supported by the BeanUtils package:

- Simple - Simple, or scalar, properties have a single value that may be retrieved or modified. The underlying property type might be a Java language primitive (such as `int`, a simple object (such as a `java.lang.String`), or a more complex object whose class is defined either by the Java language, by the application, or by a class library included with the application.
- Indexed - An indexed property stores an ordered collection of objects (all of the same type) that can be individually accessed by an integer-valued, non-negative index (or subscript). Alternatively, the entire set of values may be set or retrieved using an array. As an extension to the JavaBeans specification, the BeanUtils package considers any property whose underlying data type is `java.util.List` (or an implementation of List) to be indexed as well.
- Mapped - As an extension to standard JavaBeans APIs, the BeanUtils package considers any property whose underlying value is a `java.util.Map` to be "mapped". You can set and retrieve individual values via a String-valued key.

A variety of API methods are provided in the PropertyUtils class to get and set property values of all of these types. In the code fragments below, assume that there are two bean classes defined with the following method signatures:

        public class Employee {
            public Address getAddress(String type);
            public void setAddress(String type, Address address);
            public Employee getSubordinate(int index);
            public void setSubordinate(int index, Employee subordinate);
            public String getFirstName();
            public void setFirstName(String firstName);
            public String getLastName();
            public void setLastName(String lastName);
        }

### Basic Property Access

Getting and setting simple property values is, well, simple :-). Check out the following API signatures in the Javadocs:

- PropertyUtils.getSimpleProperty(Object bean, String name)
- PropertyUtils.setSimpleProperty(Object bean, String name, Object value)

Using these methods, you might dynamically manipulate the employee's name in an application:

        Employee employee = ...;
        String firstName = (String)
          PropertyUtils.getSimpleProperty(employee, "firstName");
        String lastName = (String)
          PropertyUtils.getSimpleProperty(employee, "lastName");
        ... manipulate the values ...
        PropertyUtils.setSimpleProperty(employee, "firstName", firstName);
        PropertyUtils.setSimpleProperty(employee, "lastName", lastName);

For indexed properties, you have two choices - you can either build a subscript into the "property name" string, using square brackets, or you can specify the subscript in a separate argument to the method call:

- PropertyUtils.getIndexedProperty(Object bean, String name)
- PropertyUtils.getIndexedProperty(Object bean, String name, int index)
- PropertyUtils.setIndexedProperty(Object bean, String name, Object value)
- PropertyUtils.setIndexedProperty(Object bean, String name, int index, Object value)

Only integer constants are allowed when you add a subscript to the property name. If you need to calculate the index of the entry you wish to retrieve, you can use String concatenation to assemble the property name expression. For example, you might do either of the following:

        Employee employee = ...;
        int index = ...;
        String name = "subordinate[" + index + "]";
        Employee subordinate = (Employee)
          PropertyUtils.getIndexedProperty(employee, name);

        Employee employee = ...;
        int index = ...;
        Employee subordinate = (Employee)
          PropertyUtils.getIndexedProperty(employee, "subordinate", index);

In a similar manner, there are two possible method signatures for getting and setting mapped properties. The difference is that the extra argument is surrounded by parentheses ("(" and ")") instead of square brackets, and it is considered to be a String-value key used to get or set the appropriate value from an underlying map.

- PropertyUtils.getMappedProperty(Object bean, String name)
- PropertyUtils.getMappedProperty(Object bean, String name, String key)
- PropertyUtils.setMappedProperty(Object bean, String name, Object value)
- PropertyUtils.setMappedProperty(Object bean, String name, String key, Object value)

You can, for example, set the employee's home address in either of these two manners:

        Employee employee = ...;
        Address address = ...;
        PropertyUtils.setMappedProperty(employee, "address(home)", address);

        Employee employee = ...;
        Address address = ...;
        PropertyUtils.setMappedProperty(employee, "address", "home", address);

### Nested Property Access

In all of the examples above, we have assumed that you wished to retrieve the value of a property of the bean being passed as the first argument to a PropertyUtils method. However, what if the property value you retrieve is really a Java object, and you wish to retrieve a property of that object instead?

For example, assume we really wanted the `city` property of the employee's home address. Using standard Java programming techniques for direct access to the bean properties, we might write:

        String city = employee.getAddress("home").getCity();

The equivalent mechanism using the PropertyUtils class is called nested property access. To use this approach, you concatenate together the property names of the access path, using "." separators -- very similar to the way you can perform nested property access in JavaScript.

- PropertyUtils.getNestedProperty(Object bean, String name)
- PropertyUtils.setNestedProperty(Object bean, String name, Object value)

The PropertyUtils equivalent to the above Java expression would be:

        String city = (String)
          PropertyUtils.getNestedProperty(employee, "address(home).city");

Finally, for convenience, PropertyUtils provides method signatures that accept any arbitrary combination of simple, indexed, and mapped property access, using any arbitrary level of nesting:

- PropertyUtils.getProperty(Object bean, String name)
- PropertyUtils.setProperty(Object bean, String name, Object value)

which you might use like this:

        Employee employee = ...;
        String city = (String) PropertyUtils.getProperty(employee,
          "subordinate[3].address(home).city");

# Dynamic Beans (DynaBeans)

### Background

The PropertyUtils class described in the preceding section is designed to provide dynamic property access on existing JavaBean classes, without modifying them in any way. A different use case for dynamic property access is when you wish to represent a dynamically calculated set of property values as a JavaBean, but without having to actually write a Java class to represent these properties. Besides the effort savings in not having to create and maintain a separate Java class, this ability also means you can deal with situations where the set of properties you care about is determined dynamically (think of representing the result set of an SQL select as a set of JavaBeans ...).

To support this use case, the BeanUtils package provides the DynaBean interface, which must be implemented by a bean class actually implementing the interface's methods, and the associated DynaClass interface that defines the set of properties supported by a particular group of DynaBeans, in much the same way that `java.lang.Class` defines the set of properties supported by all instances of a particular JavaBean class.

For example, the `Employee` class used in the examples above might be implemented as a DynaBean, rather than as a standard JavaBean. You can access its properties like this:

        DynaBean employee = ...; // Details depend on which
                                 // DynaBean implementation you use
        String firstName = (String) employee.get("firstName");
        Address homeAddress = (Address) employee.get("address", "home");
        Object subordinate = employee.get("subordinate", 2);

One very important convenience feature should be noted: the PropertyUtils property getter and setter methods understand how to access properties in DynaBeans. Therefore, if the bean you pass as the first argument to, say, `PropertyUtils.getSimpleProperty()` is really a DynaBean implementation, the call will get converted to the appropriate DynaBean getter method transparently. Thus, you can base your application's dynamic property access totally on the PropertyUtils APIs, if you wish, and use them to access either standard JavaBeans or DynaBeans without having to care ahead of time how a particular bean is implemented.

Because DynaBean and DynaClass are interfaces, they may be implemented multiple times, in different ways, to address different usage scenarios. The following subsections describe the implementations that are provided as a part of the standard BeanUtils package, although you are encouraged to provide your own custom implementations for cases where the standard implementations are not sufficient.

### `BasicDynaBean` and `BasicDynaClass`

The BasicDynaBean and BasicDynaClass implementation provides a basic set of dynamic property capabilities where you want to dynamically define the set of properties (described by instances of DynaProperty). You start by defining the DynaClass that establishes the set of properties you care about:

        DynaProperty[] props = new DynaProperty[]{
            new DynaProperty("address", java.util.Map.class),
            new DynaProperty("subordinate", mypackage.Employee[].class),
            new DynaProperty("firstName", String.class),
            new DynaProperty("lastName",  String.class)
          };
        BasicDynaClass dynaClass = new BasicDynaClass("employee", null, props);

Note that the 'dynaBeanClass' argument (in the constructor of `BasicDynaClass`) can have the value of `null`. In this case, the value of `dynaClass.getDynaBeanClass` will just be the `Class` for BasicDynaBean.

Next, you use the `newInstance()` method of this DynaClass to create new DynaBean instances that conform to this DynaClass, and populate its initial property values (much as you would instantiate a new standard JavaBean and then call its property setters):

        DynaBean employee = dynaClass.newInstance();
        employee.set("address", new HashMap());
        employee.set("subordinate", new mypackage.Employee[0]);
        employee.set("firstName", "Fred");
        employee.set("lastName", "Flintstone");

Note that the DynaBean class was declared to be `DynaBean` instead of `BasicDynaBean`. In general, if you are using DynaBeans, you will not want to care about the actual implementation class that is being used -- you only care about declaring that it is a `DynaBean` so that you can use the DynaBean APIs.

As stated above, you can pass a DynaBean instance as the first argument to a `PropertyUtils` method that gets and sets properties, and it will be interpreted as you expect -- the dynamic properties of the DynaBean will be retrieved or modified, instead of underlying properties on the actual BasicDynaBean implementation class.

### `ResultSetDynaClass` (Wraps ResultSet in DynaBeans)

A very common use case for DynaBean APIs is to wrap other collections of "stuff" that do not normally present themselves as JavaBeans. One of the most common collections that would be nice to wrap is the `java.sql.ResultSet` that is returned when you ask a JDBC driver to perform a SQL SELECT statement. Commons BeanUtils offers a standard mechanism for making each row of the result set visible as a DynaBean, which you can utilize as shown in this example:

      Connection conn = ...;
      Statement stmt = conn.createStatement();
      ResultSet rs = stmt.executeQuery
        ("select account_id, name from customers");
      Iterator rows = (new ResultSetDynaClass(rs)).iterator();
      while (rows.hasNext()) {
        DynaBean row = (DynaBean) rows.next();
        System.out.println("Account number is " +
                           row.get("account_id") +
                           " and name is " + row.get("name"));
      }
      rs.close();
      stmt.close();

### `RowSetDynaClass` (Disconnected ResultSet as DynaBeans)

Although `ResultSetDynaClass` is a very useful technique for representing the results of an SQL query as a series of DynaBeans, an important problem is that the underlying `ResultSet` must remain open throughout the period of time that the rows are being processed by your application. This hinders the ability to use `ResultSetDynaClass` as a means of communicating information from the model layer to the view layer in a model-view-controller architecture such as that provided by the Struts Framework, because there is no easy mechanism to assure that the result set is finally closed (and the underlying `Connection` returned to its connection pool, if you are using one).

The `RowSetDynaClass` class represents a different approach to this problem. When you construct such an instance, the underlying data is copied into a set of in-memory DynaBeans that represent the result. The advantage of this technique, of course, is that you can immediately close the ResultSet (and the corresponding Statement), normally before you even process the actual data that was returned. The disadvantage, of course, is that you must pay the performance and memory costs of copying the result data, and the result data must fit entirely into available heap memory. For many environments (particularly in web applications), this tradeoff is usually quite beneficial.

As an additional benefit, the `RowSetDynaClass` class is defined to implement `java.io.Serializable`, so that it (and the DynaBeans that correspond to each row of the result) can be conveniently serialized and deserialized (as long as the underlying column values are also Serializable). Thus, `RowSetDynaClass` represents a very convenient way to transmit the results of an SQL query to a remote Java-based client application (such as an applet).

The normal usage pattern for a `RowSetDynaClass` will look something like this:

        Connection conn = ...;  // Acquire connection from pool
        Statement stmt = conn.createStatement();
        ResultSet rs = stmt.executeQuery("SELECT ...");
        RowSetDynaClass rsdc = new RowSetDynaClass(rs);
        rs.close();
        stmt.close();
        ...;                    // Return connection to pool
        List rows = rsdc.getRows();
        ...;                   // Process the rows as desired

### `WrapDynaBean` and `WrapDynaClass`

OK, you've tried the DynaBeans APIs and they are cool -- very simple `get()` and `set()` methods provide easy access to all of the dynamically defined simple, indexed, and mapped properties of your DynaBeans. You'd like to use the DynaBean APIs to access all of your beans, but you've got a bunch of existing standard JavaBeans classes to deal with as well. This is where the WrapDynaBean (and its associated WrapDynaClass) come into play. As the name implies, a WrapDynaBean is used to "wrap" the DynaBean APIs around an existing standard JavaBean class. To use it, simply create the wrapper like this:

        MyBean bean = ...;
        DynaBean wrapper = new WrapDynaBean(bean);
        String firstName = wrapper.get("firstName");

Note that, although appropriate `WrapDynaClass` instances are created internally, you never need to deal with them.

### Lazy DynaBeans (LazyDynaBean, LazyDynaMap and LazyDynaClass)

You bought into the DynaBeans because it saves coding all those POJO JavaBeans but you're here because lazy caught your eye and wondered whats that about? What makes these flavors of DynaBean lazy are the following features:

- Lazy property addition - lazy beans use a `DynaClass` which implements the `MutableDynaClass` interface. This provides the ability to add and remove a DynaClass's properties. Lazy beans use this feature to automatically add a property which doesn't exist to the DynaClass when the `set(name, value)` method is called.
- Lazy List/Array growth - If an indexed property is not large enough to accomodate the `index` being set then the `List` or `Array` is automatically grown so that it is.
- Lazy List/Array instantiation - if an indexed property doesn't exist then calling the DynaBean's indexed property getter/setter methods (i.e. `get(name, index)` or `set(name, index, value)`) results in either a new `List` or `Array` being instantiated. If the indexed property has not been defined in the DynaClass then it is automatically added and a default `List` implementation instantiated.
- Lazy Map instantiation - if a mapped property doesn't exist then calling the DynaBean's mapped property getter/setter methods (i.e. `get(name, key)` or `set(name, key, value)`) results in a new `Map` being instantiated. If the mapped property has not been defined in the DynaClass then it is automatically added and a default `Map` implementation instantiated.
- Lazy Bean instantiation - if a property is defined in the `DynaClass` as a `DynaBean` or regular bean and doesn't exist in the `DynaBean` then `LazyDynaBean` wiill try to instantiate the bean using a default empty constructor.

LazyDynaBean is the standard lazy bean implementation. By default it is associated with a LazyDynaClass which implements the MutableDynaClass interface - however it can be used with any `MutableDynaClass` implementation. The question is how do I use it? - well it can be as simple as creating a new bean and then calling the getters/setters...

        DynaBean dynaBean = new LazyDynaBean();

        dynaBean.set("foo", "bar");                   // simple

        dynaBean.set("customer", "title", "Mr");      // mapped
        dynaBean.set("customer", "surname", "Smith"); // mapped

        dynaBean.set("address", 0, addressLine1);     // indexed
        dynaBean.set("address", 1, addressLine2);     // indexed
        dynaBean.set("address", 2, addressLine3);     // indexed

LazyDynaMap is a light wieght `DynaBean` facade to a `Map` with all the usual lazy features. Its light weight because it doesn't have an associated `DynaClass` containing all the properties. In fact it actually implements the `DynaClass` interface itself (and `MutableDynaClass`) and derives all the DynaClass information from the actual contents of the `Map`. A `LazyDynaMap` can be created around an existing `Map` or can instantiate its own `Map`. After any `DynaBean` processing has finished the `Map` can be retrieved and the DynaBean facade discarded.

If you need a new `Map` then to use....

        DynaBean dynaBean = new LazyDynaMap();        // create DynaBean

        dynaBean.set("foo", "bar");                   // simple
        dynaBean.set("customer", "title", "Mr");      // mapped
        dynaBean.set("address", 0, addressLine1);     // indexed

        Map myMap = dynaBean.getMap()                 // retrieve the Map

or to use with an existing `Map` ....

        Map myMap = ....                             // exisitng Map
        DynaBean dynaBean = new LazyDynaMap(myMap);  // wrap Map in DynaBean
        dynaBean.set("foo", "bar");                  // set properties

LazyDynaClass extends BasicDynaClass and implements the MutableDynaClass interface. It can be used with other `DynaBean` implementations, but it is the default `DynaClass` used by `LazyDynaBean`. When using the `LazyDynaBean` there may be no need to have anything to do with the `DynaClass` However sometimes there is a requirement to set up the `DynaClass` first - perhaps to define the type of array for an indexed property, or if using the DynaBean in restricted mode (see note below) is required. Doing so is straight forward...

Either create a `LazyDynaClass` first...

        MutableDynaClass dynaClass = new LazyDynaClass();    // create DynaClass

        dynaClass.add("amount", java.lang.Integer.class);    // add property
        dynaClass.add("orders", OrderBean[].class);          // add indexed property
        dynaClass.add("orders", java.util.TreeMapp.class);   // add mapped property

        DynaBean dynaBean = new LazyDynaBean(dynaClass);     // Create DynaBean with associated DynaClass

or create a `LazyDynaBean` and get the `DynaClass`...

        DynaBean dynaBean = new LazyDynaBean();              // Create LazyDynaBean
        MutableDynaClass dynaClass =
                 (MutableDynaClass)dynaBean.getDynaClass();  // get DynaClass

        dynaClass.add("amount", java.lang.Integer.class);    // add property
        dynaClass.add("myBeans", myPackage.MyBean[].class);  // add 'array' indexed property
        dynaClass.add("myMap", java.util.TreeMapp.class);    // add mapped property

NOTE: One feature of MutableDynaClass is that it has a Restricted property. When the DynaClass is restricted no properties can be added or removed from the `DynaClass`. Neither the `LazyDynaBean` or `LazyDynaMap` will add properties automatically if the `DynaClass` is restricted.

# Data Type Conversions

### Background

So far, we've only considered the cases where the data types of the dynamically accessed properties are known, and where we can use Java casts to perform type conversions. What happens if you want to automatically perform type conversions when casting is not possible? The BeanUtils package provides a variety of APIs and design patterns for performing this task as well.

### `BeanUtils` and `ConvertUtils` Conversions

A very common use case (and the situation that caused the initial creation of the BeanUtils package) was the desire to convert the set of request parameters that were included in a `javax.servlet.HttpServletRequest` received by a web application into a set of corresponding property setter calls on an arbitrary JavaBean. (This is one of the fundamental services provided by the Struts Framework, which uses BeanUtils internally to implement this functionality.)

In an HTTP request, the set of included parameters is made available as a series of String (or String array, if there is more than one value for the same parameter name) instances, which need to be converted to the underlying data type. The BeanUtils class provides property setter methods that accept String values, and automatically convert them to appropriate property types for Java primitives (such as `int` or `boolean`), and property getter methods that perform the reverse conversion. Finally, a `populate()` method is provided that accepts a `java.util.Map` containing a set of property values (keyed by property name), and calls all of the appropriate setters whenever the underlying bean has a property with the same name as one of the request parameters. So, you can perform the all-in-one property setting operation like this:

        HttpServletRequest request = ...;
        MyBean bean = ...;
        HashMap map = new HashMap();
        Enumeration names = request.getParameterNames();
        while (names.hasMoreElements()) {
          String name = (String) names.nextElement();
          map.put(name, request.getParameterValues(name));
        }
        BeanUtils.populate(bean, map);

The `BeanUtils` class relies on conversion methods defined in the ConvertUtils class to perform the actual conversions, and these methods are availablve for direct use as well. WARNING - It is likely that the hard coded use of `ConvertUtils` methods will be deprecated in the future, and replaced with a mechanism that allows you to plug in your own implementations of the Converter interface instead. Therefore, new code should not be written with reliance on ConvertUtils.

### Defining Your Own Converters

The `ConvertUtils` class supports the ability to define and register your own String --> Object conversions for any given Java class. Once registered, such converters will be used transparently by all of the `BeanUtils` methods (including `populate()`). To create and register your own converter, follow these steps:

- Write a class that implements the Converter interface. The `convert()` method should accept the `java.lang.Class` object of your application class (i.e. the class that you want to convert to, and a String representing the incoming value to be converted.
- At application startup time, register an instance of your converter class by calling the `ConvertUtils.register()` method.

### Locale Aware Conversions

The standard classes in `org.apache.commons.beanutils` are not locale aware. This gives them a cleaner interface and makes then easier to use in situations where the locale is not important.

Extended, locale-aware analogues can be found in `org.apache.commons.beanutils.locale` . These are built along the same lines as the basic classes but support localization.

# Utility Objects And Static Utility Classes

### Background

So far, the examples have covered the static utility classes (`BeanUtils`, `ConvertUtils` and `PropertyUtils`). These are easy to use but are somewhat inflexible. These all share the same registered converters and the same caches.

This functionality can also be accessed through utility objects (in fact, the static utility class use worker instances of these classes). For each static utility class, there is a corresponding class with the same functionality that can be instantiated:

Static Utility Class | Utility Object
--- | ---
BeanUtils | BeanUtilsBean
ConvertUtils | ConvertUtilsBean
PropertyUtils | PropertyUtilsBean

Creating an instances allow gives guarenteed control of the caching and registration to the code that creates it.

# Collections

Note: these classes depend on Commons Collection 3.0 or higher and are optional. To use them, please ensure that this library is available and that either the all-in-one jar is used or the modular `commons-beanutils-bean-collections.jar` is available (in addition to `commons-beanutils-core.jar`, of course :-).

### Comparing Beans

`org.apache.commons.beanutils.BeanComparator` is a `Comparator` implementation that compares beans based on a shared property value.

### Operating On Collections Of Beans

The `Closure` interface in `commons-collections` encapsulates a block of code that executes on an arbitrary input Object. `Commons-collections` contains code that allows `Closures` to be applied to the contents of a Collection. For more details, see the commons-collections documentation.

`BeanPropertyValueChangeClosure` is a `Closure` that sets a specified property to a particular value. A typical usage is to combine this with `commons-collections` so that all the beans in a collection can have a particular property set to a particular value.

For example, set the activeEmployee property to TRUE for an entire collection: ``

        // create the closure
        BeanPropertyValueChangeClosure closure =
            new BeanPropertyValueChangeClosure( "activeEmployee", Boolean.TRUE );

        // update the Collection
        CollectionUtils.forAllDo( peopleCollection, closure );

### Querying Or Filtering Collections Of Beans

The `Predicate` interface in `commons-collections` encapsulates an evaluation of an input Object that returns either true or false. `Commons-collections` contains code that allows `Predicates` to be applied to be used to filter collections. For more details, see the commons-collections documentation.

`BeanPropertyValueEqualsPredicate` is a `Predicate` that evaluates a set property value against a given value. A typical usage is (in combination with `commons-collections`) to filter collections on the basis of a property value.

For example, to filter a collection to find all beans where active employee is false use: ``

        BeanPropertyValueEqualsPredicate predicate =
            new BeanPropertyValueEqualsPredicate( "activeEmployee", Boolean.FALSE );

        // filter the Collection
        CollectionUtils.filter( peopleCollection, predicate );

### Transforming Collections Of Beans

The `Transformer` interface in `commons-collections` encapsulates the transformation of an input Object into an output object. `Commons-collections` contains code that allows `Transformers` to be applied produce a collection of outputs from a collection of inputs. For more details, see the commons-collections documentation.

`BeanToPropertyTransformer` is a `Transformer` implementation that transforms a bean into it's property value.

For example, to find all cities that are contained in the address of each person property of each bean in a collection: ``

        // create the transformer
        BeanToPropertyValueTransformer transformer = new BeanToPropertyValueTransformer( "person.address.city" );

        // transform the Collection
        Collection peoplesCities = CollectionUtils.collect( peopleCollection, transformer );

# Frequently Asked Questions

### Why Can't BeanUtils Find My Method?

The BeanUtils package relies on introspection rather than reflection. This means that it will find only JavaBean compliant properties.

There are some subtleties of this specification that can catch out the unwary:

- A property can have only one set and one get method. Overloading is not allowed.
- The `java.beans.Introspector` searches widely for a custom BeanInfo class. If your class has the same name as another with a custom BeanUtils (typically a java API class) then the `Introspector` may use that instead of creating via reflection based on your class. If this happens, the only solution is to create your own BeanInfo.

### How Do I Set The BeanComparator Order To Be Ascending/Descending?

BeanComparator relies on an internal Comparator to perform the actual comparisions. By default, `org.apache.commons.collections.comparators.ComparableComparator` is used which imposes a natural order. If you want to change the order, then a custom Comparator should be created and passed into the appropriate constructor.

For example:

`

        import org.apache.commons.collections.comparators.ComparableComparator;
        import org.apache.commons.collections.comparators.ReverseComparator;
        import org.apache.commons.beanutils.BeanComparator;
        ...
        BeanComparator reversedNaturalOrderBeanComparator
            = new BeanComparator("propertyName", new ReverseComparator(new ComparableComparator()));
        Collections.sort(myList, reversedNaturalOrderBeanComparator);
        ...

`

# Packaging

BeanUtils now comes packaged (into jars) in two different ways:

1. an all-in-one jar (`commons-beanutils.jar`)
2. and as modular component jars:
   - `commons-beanutils-core.jar` (core classes)
   - `commons-beanutils-bean-collections.jar` (additional dependency on commons-collections 3.0)

Those who want it all should grab the all-in-one `commons-beanutils.jar` (and remember to add any optional dependencies needed) whereas those who need minimal dependencies should use `commons-beanutils-core.jar` plus any optional jars they plan to use.

The binary distribution contains all these jars.