ORIGIN

IOC Manage Bean Based on XML

Spring6IOC Bean XML 16 mins2.6k words

This is note of learning IOC management.

Get Bean

Create an user object in xml

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<bean id="user" class="com.yao.spring6.User"></bean>

where id is the identify name and class is full package name + Class name

Use ApplicationoContext to get the xml file.

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ApplicationContext context = new ClassPathXmlApplicationContext("beam.xml");

There are three ways to get the object.

  1. by id

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    User user = (User) context.getBean("user");

  2. by type

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    User user = context.getBean(User.class);

    There must be only one object defined in XML in which the class type is User.

  3. by id and type

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    User user = context.getBean("user", User.class);

If defines an interface and want to get the bean object by interface type, there should only be one bean tag in xml that implements the interface.

dependency injection

How to fill the values after we get the object?

  1. Using set method

    Write setXX() method in each class for each property. And then call them after get a new object.

  2. using constructor

    Write constructor with parameter that fill each property.

So how do we do it in Spring?

Set method

First we need to generate setters for our class

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package org.example;

public class Book {
    String bookName;
    String author;
    
    public void setBookName(String bookName) {
        this.bookName = bookName;
    }

    public void setAuthor(String author) {
        this.author = author;
    }
}

Then the xml is wrote like this:

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<bean id="book" class="org.example.Book">
    <property name="bookName" value="三体"></property>
    <property name="author" value="刘慈欣"></property>
</bean>

Name is the name of each property.

Then we can get the object by ApplicationContext.

Constructor

First we need to have a constructor function

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public class Book {
    String bookName;
    String author;

    public Book(String bookName, String author) {
        this.bookName = bookName;
        this.author = author;
    }
}

Then in xml, we write:

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<bean id="bookCon" class="org.example.Book">
    <constructor-arg name="bookName" value="三体"></constructor-arg>
    <constructor-arg name="author" value="刘慈欣"></constructor-arg>
</bean>

name is the name of each property.

There are four different types of value.

  1. variable, just use value to set the value

  2. NULL, use <null></null> or <null/> inside to present a null value

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    <bean id="bookCon" class="org.example.Book">
        <property name="bookName" value="三体"></property>
        <property name="author">
            <null></null>
        </property>
    </bean>

  3. if the value contains xml code, use escape character

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    <bean id="bookCon" class="org.example.Book">
        <property name="bookName" value="&lt; &lg;"></property>
    </bean>

  4. CDATA

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    <bean id="bookCon" class="org.example.Book">
        <property name="bookName">
            <value><![CDATA[a < b]]></value>
        </property>
    </bean>

Special type property

Object

Assume we have two class, and one class use other class as a property

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public class Department {
    private String name;
    // some set methods...
}
public class Employee {
    private int age;
    private String name;
    private Department dep;
    // some set methods...
}

outer bean

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<bean id="department" class="org.example.di.Department">
    <property name="name" value="development"></property>
</bean>

<bean id="employee" class="org.example.di.Employee">
    <property name="name" value="Jonn"></property>
    <property name="age" value="25"></property>
    <!-- Department class value type -->
    <property name="dep" ref="department"></property>
</bean>

Use ref as a reference to other bean tag. And value of ref is the id of another bean.

inner bean

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<bean id="employee2" class="org.example.di.Employee">
    <property name="name" value="Marry"></property>
    <property name="age" value="18"></property>
    <!-- Department class value type -->
    <property name="dep">
        <bean id="department2" class="org.example.di.Department">
            <property name="name" value="finance"></property>
        </bean>
    </property>
</bean>

Cascade assignment

you can change the value of the object type property.

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<bean id="department3" class="org.example.di.Department">
    <property name="name" value="development"></property>
</bean>

<bean id="employee3" class="org.example.di.Employee">
    <property name="name" value="Jonn"></property>
    <property name="age" value="25"></property>
    <property name="dep" ref="department3"></property>
    <!-- change value of object type -->
    <property name="dep.name" value="sale"></property>
</bean>

Array

Let’s add an array to the Employee and its setter function.t

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public class Employee {
    private int age;
    private String name;
    private Department dep;
    private String[] hobbies;
}

So we use <array></array> tag to set array value.

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<bean id="employee" class="org.example.di.Employee">
    ...
    <property name="hobbies">
        <array>
            <value>eat</value>
            <value>sleep</value>
            <value>play</value>
        </array>
    </property>
</bean>

List Array

Add an List to Department and its setter function.

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public class Department {
    private String name;
    private List<Employee> employeeList;
}

Since the type of List is an object, we need to create multiple bean tags

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<bean id="empone" class="org.example.di.Employee">
    <property name="name" value="Jonn"></property>
    <property name="age" value="25"></property>
</bean>

<bean id="emptwo" class="org.example.di.Employee">
    <property name="name" value="Sandy"></property>
    <property name="age" value="26"></property>
</bean>

<bean id="department" class="org.example.di.Department">
    <property name="name" value="development"></property>
    <property name="employeeList">
        <list>
            <ref bean="empone"></ref>
            <ref bean="emptwo"></ref>
        </list>
    </property>
</bean>

Map

Change to two simple class

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public class Student {
    private String id;
    private String name;
    private Map<String, Teacher> teacherMap;
    private List<String> course;
}
public class Teacher {
    private String id;
    private String name;
}
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<bean id="teacher" class="org.example.dimap.Teacher">
    <property name="id" value="101"></property>
    <property name="name" value="Lucy"></property>
</bean>

<bean id="student" class="org.example.dimap.Student">
    <property name="id" value="2051"></property>
    <property name="name" value="Sam"></property>
    <property name="teacherMap">
        <map>
            <entry>
                <key>
                    <value>Math</value>
                </key>
                <ref bean="teacher"></ref>
            </entry>
        </map>
    </property>
</bean>

List Type Bean

add another Lesson Class

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public class Lesson {
    private String name;
}
public class Student {
    private String id;
    private String name;
    private Map<String, Teacher> teacherMap;
    private List<Lesson> course;
}

First, we need to add namespace util to the xml file

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<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:util="http://www.springframework.org/schema/util"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
       http://www.springframework.org/schema/beans/spring-beans.xsd
       http://www.springframework.org/schema/util
       http://www.springframework.org/schema/util/spring-util.xsd">
</beans>

Then we add some Lessons and some Teachers

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<bean id="lesson1" class="org.example.dimap.Lesson">
    <property name="name" value="Math"></property>
</bean>
<bean id="lesson2" class="org.example.dimap.Lesson">
    <property name="name" value="Python Lesson"></property>
</bean>

<bean id="teacher1" class="org.example.dimap.Teacher">
    <property name="id" value="101"></property>
    <property name="name" value="Lucy"></property>
</bean>

<bean id="teacher2" class="org.example.dimap.Teacher">
    <property name="id" value="546"></property>
    <property name="name" value="Tom"></property>
</bean>

Then add the <util:xxx></util:xxx> tag and reference it in student bean tag.

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<util:list id="lessonList">
    <ref bean="lesson1"></ref>
    <ref bean="lesson2"></ref>
</util:list>

<util:map id="tchrMap">
    <entry>
        <key>
            <value>Math</value>
        </key>
        <ref bean="teacher1"></ref>
    </entry>
    <entry>
        <key>
            <value>Python Lesson</value>
        </key>
        <ref bean="teacher2"></ref>
    </entry>
</util:map>

<bean id="student" class="org.example.dimap.Student">
    <property name="id" value="2051"></property>
    <property name="name" value="Sam"></property>
    <property name="course" ref="lessonList"></property>
    <property name="teacherMap" ref="tchrMap"></property>
</bean>

P namespace

P namespace is to avoid conflicts among property names.

it write in the header of xml file like this

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<beans ...
       xmlns:p="http://www.springframework.org/schema/p"
       xsi:schemaLocation=" ... ">
</beans>

Then use p tag property like this:

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<bean id="student" class="org.example.dimap.Student"
      p:id="2051" p:name="Sam" p:course-ref="lessonList" p:teacherMap-ref="tchrMap">
</bean>

Using outer property file

Add dependencies

add MySQL and connection pool dependencies.

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<dependencies>
    <dependency>
        <groupId>mysql</groupId>
        <artifactId>mysql-connector-java</artifactId>
        <version>8.0.29</version>
    </dependency>

    <dependency>
        <groupId>com.alibaba</groupId>
        <artifactId>druid</artifactId>
        <version>1.2.16</version>
    </dependency>
</dependencies>

Properties file

create the file under source folder.

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user=root
password=123
url=jdbc:mysql://localhost:3306/test
driverClass=com.mysql.jdbc.Driver

Xml bean file

Create a new xml file under source folder and add context namespace

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<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:context="http://www.springframework.org/schema/context"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
       http://www.springframework.org/schema/beans/spring-beans.xsd
       http://www.springframework.org/schema/context
       http://www.springframework.org/schema/context/spring-context.xsd">

</beans>

And then we need to introduce the properties file

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<context:property-placeholder location="classpath:jdbc.properties"></context:property-placeholder>

Then add connection pool in xml

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<bean id="connectionPool" class="com.alibaba.druid.pool.DruidDataSource">
    <property name="url" value="${url}"></property>
    <property name="password" value="${password}"></property>
    <property name="username" value="${user}"></property>
    <property name="driverClassName" value="${driverClass}"></property>
</bean>

Scope of bean

Use scope in bean tag can control whether the instance is singleton or prototype.

value meaning when to create
singleton object related to the bean is only one in IOC when initializing IOC
prototype multiple object related to this bean in IOC when getting bean 
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<bean id="" class="" scope="singleton"></bean>

is scope is not specified, default value is singleton.

Life cycle of bean

  1. create bean object (call parameterless constructor)
  2. set properties for bean object
  3. bean post processor(before initialization)
  4. initialize bean object (init-method="xxxx")
  5. bean post processor(after initialization)
  6. bean object create finished
  7. destroy bean object(destroy-method="xxxx") and use context.close() to call the destroy method.
  8. close IOC

FactoryBean

If the class is an implement of FactoryBean, then creating the object in xml will get the the object that getObject() returns.

A FactoryBean is a pattern to encapsulate interesting object construction logic in a class.

Auto load based on xml

If Class has a property that is the type of Class B, then when write it in bean, setting autowire will automatically find its referenced instance by Type or by Name.

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<bean id="" class="xxx.xxx.B"></bean>
<bean id="" class="xxx.xxx.A" autowire="byType"></bean>

ByType

If can’t find the suitable bean, then will set this property to null.

If find many beans that suits, then will throw exception: NoUniqueBeanDefininationException.

ByName

the property name should keep same with the bean id.

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