setter injection bean patterns
Resin configures beans using setter injection
patterns, supporting the Inversion-of-Control design pattern.
A "bean" is any plain-old Java object which follows standard
configuration patterns. Because Resin can find the bean-style setters
from looking at the class, it can configure those setters in a
configuration file like the web.xml.
Resin's configuration follows the Assembly Line
or Dependency Injection pattern.
The Assembly Line pattern gives configuration responsibility to the
container where it belongs, while keeping the application code independent
of the container. Bean-style configuration setters for simple properties
form the foundation for the Assembly Line pattern. If an application
follows the bean patterns, it can be configuration in any container following
the Assembly Line (setter injection) pattern.
We strongly recommend following the Assembly Line pattern throughout
an application, even if your application does not use Resin to configure
itself. Following the Assembly Line pattern makes application code
easier to understand, maintain, configure and test.
Property Configuration: setXXX
The bean configuration form the foundation of the Assembly
Line pattern. Since most applications already follow the bean patterns,
they get property configuration with no changes.
Each configuration parameter foo has a corresponding setter method
setFoo with a single argument for the value. Resin
looks at the class using Java's reflection to find the setFoo
method.
Bean-style configuration for a single value setter
<init>
<greeting>Hello, World!</greeting>
<another-greeting>Hello, Mom!</another-greeting>
</init>
Bean-style java code for a single value setter
public class MyBean {
private String _greeting;
private String _anotherGreeting;
public void setGreeting(String greeting)
{
_greeting = greeting;
}
public void setAnotherGreeting(String anotherGreeting)
{
_anotherGreeting = anotherGreeting;
}
}
Setter injection connects resources following the same
bean-style setter pattern. Where bean properties configure simple
values like strings and integers, setter injection configures
other resources like databases and application components.
Resin uses JNDI to store the intermediate resources, e.g. storing
a database in java:comp/env/jdbc/test. The configuration file
specifies the JNDI resource using the JSP expression language and
jndi:lookup.
Configuration for Setter Injection
<init>
<data-source>\${jndi:lookup("jdbc/test")}<data-source>
</init>
Setter Injection for a DataSource
public class MyBean {
private DataSource _dataSource;
public void setDataSource(DataSource ds)
{
_dataSource = ds;
}
}
Compatibility
Setter injection is portable to containers which
support dependency injection.
Container Properties: addXXX
Resources often act as containers for lists of values and map values.
The addXXX pattern adds multiple values for a single property.
A setter method addFoo allows multiple values to be
specified from the configuration.
Bean-style configuration for setting multiple values
<init>
<greeting>Hello, World!</greeting>
<greeting>Hello, Mom!</greeting>
</init>
Bean-style java code for setting multiple values
public class MyBean {
private LinkedList _greetings = new LinkedList();
public void addGreeting(String greeting)
{
_greetings.add(greeting);
}
}
Validation and Assembly: @PostConstruct
Well-written resources will validate their configuration and may
perform additional assembly tasks. Resin calls @PostConstruct methods
after all the setter methods have been called.
Bean-style @PostConstruct
public class MyBean {
private String _language;
private String _country;
Locale locale;
public void setLanguage(String language)
{
_language = language;
}
public void setCountry(int country)
{
_country = country;
}
@PostConstruct
public void init()
{
locale = new Locale(language, country);
}
}
Validation Exceptions
If an exception is thrown from any of the methods in the bean,
Resin will attach a file name and line number that correspond to the
configuration file.
Bean-style exceptions
public class MyBean {
private String _language;
private String _country;
Locale _locale;
public void setLanguage(String language)
throws Exception
{
if (language.length() != 2)
throw new Exception("'language' must be a two-character string");
_language = language;
}
public void setCountry(int country)
throws Exception
{
if (country.length() != 2)
throw new Exception("'country' must be a two-character string");
_country = country;
}
@PostConstruct
public void init()
{
if (_country == null)
throw new Exception("'country' is required");
if (_language == null)
throw new Exception("'language' is required");
_locale = new Locale(language,country);
}
}
500 Servlet Exception
WEB-INF/web.xml:9: java.lang.Exception: 'country' must be a two-character string
Beans can be nested, allowing a bean to have setters that have
other sub-beans as the type.
Bean-style configuration for sub-beans
<init>
<table>
<name>Foo</name>
<timestamp-field>tstamp</timestamp-field>
</table>
<table name="Bar" timestamp-field="ts"/>
</init>
Bean-style java code for sub-beans
// a class to periodically clean old log records from the database
public class LogCleaner {
List _logTables = new LinkedList();
// the createXXX method is optional, and allows use something other than
// the default constructor for a sub-bean
public LogTable createTable()
{
return new LogTable();
}
// you could also use setTable(LogTable logTable)
public void addTable(LogTable logTable)
{
_logTables.add(logTable);
}
public class LogTable {
String _name;
String _timestampField;
public void setName(String name)
{
_name = name;
}
public void setTimestampField(String timestampField)
{
_timestampField = timestampField;
}
@PostConstruct
public void init()
throws Exception
{
if (_name == null)
throw new Exception("'name' is required");
if (_timestampField == null)
throw new Exception("'timestamp-field' is required");
}
public void cleanTable(DataSource pool)
{
Connection conn = null;
try {
conn = pool.getConnection();
...
} catch (SQLException e) {
throw new ServletException(e);
} finally {
try {
if (conn != null)
conn.close();
} catch (SQLException e) {
}
}
}
}
...
}
Setting with the body text
The addText() method will capture the body of the tag
for a bean setter.
Bean-style configuration for setting with the body text
<init>
<message>This is the message</message>
</init>
Bean-style java code for setting with the body text
public class MyBean {
Message _msg;
public Message createMessage() { return new Message(); }
public void setMessage(Message msg) { _msg = msg; }
public class Message {
String _text;
public void addText(String text) { _text = text; }
public String getText() { return _text; }
}
}
Returning a different object
There are some unusual cases where the configured bean is just a
configuration object and you want to return a different bean. The
bean can implement a method Object replaceObject() to return a
different object. Called after the @PostConstruct methods.
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