Beans are “lazily” loaded into bean factories, meaning that while the bean factory will imme-
diately load the bean definitions

 A bean factory lazily loads all beans, deferring bean
creation  until  the  getBean()  method  is  called.  An  application  context  is  a  bit
smarter and preloads all singleton beans upon context startup. By preloading sin-
gleton beans, you ensure that they will be ready to use when needed—your appli-
cation won’t have to wait for them to be created.

Initialize beans.
If the bean implements InitializingBean, its
afterPropertiesSet() method will be called. If the bean
has a custom init method declared, the specified initialization
method will be called.

Notice that the value attribute is used exactly the same when setting a numeric
value as it is when setting a String value. Spring will determine the correct type
for  the  value  based  on  the  property’s  type.  Since  the  age  property  is  an  int,
Spring knows to convert 37 to an int value before calling setAge().

inner bean.
 <bean id="kenny"
    class="com.springinaction.springidol.Instrumentalist">
  <property name="song" value="Jingle Bells" />
  <property name="instrument">
    <bean class="org.springinaction.springidol.Saxophone" />
  </property>
</bean>

wiring collection
<list>      Wiring a list of values, allowing duplicates.
<set>       Wiring a set of values, ensuring no duplicates
<map>       Wiring a collection of name-value pairs where name and value can be of any type
<props>     Wiring a collection of name-value pairs where the name and value are both Strings

 The  key  difference  between  the  two  is  that  when  using
<props>, both the keys and values are Strings, while <map> allows keys and val-
ues of any type.

map 举例：
    <map>
      <entry key="GUITAR" value-ref="guitar" />
      <entry key="CYMBAL" value-ref="cymbal" />
      <entry key="HARMONICA" value-ref="harmonica" />
    </map

<property name="someNonNullProperty"> <null/> </property>

Spring provides four flavors of autowiring:
■      byName—Attempts to find a bean in the container whose name (or ID) is
the same as the name of the property being wired. If a matching bean is not
found, the property will remain unwired.
■      byType—Attempts  to  find  a  single  bean  in  the  container  whose  type
matches the type of the property being wired. If no matching bean is found,
the  property  will  not  be  wired.  If  more  than  one  bean  matches,  an
org.springframework.beans.factory.UnsatisfiedDependencyExcep-
tion will be thrown.
■      constructor—Tries to match up one or more beans in the container with
the  parameters  of  one  of  the  constructors  of  the  bean  being  wired.  In  the
event of ambiguous beans or ambiguous constructors, an org.springframe-
work.beans.factory.UnsatisfiedDependencyException will be thrown.
■      autodetect—Attempts  to  autowire  by  constructor  first  and  then  using
byType.  Ambiguity  is  handled  the  same  way  as  with  constructor  and
byType wiring.