因為單例類的特殊性，spring對於單例類的注入提供了factory-method屬性，先上程式碼。

OneInstance類：

package spring.ioc02;

public class OneInstance {

    public static class GetInstanceHolder{
        static OneInstance oneInstance = new OneInstance();
    }

    public static OneInstance getInstance(){
        return GetInstanceHolder.oneInstance;
    }

    public
 
 void finish(){
        System.out.println("例項化完成");
    }
}

OneInstance 類裡面有個內部類GetInstanceHolder，它有一個靜態屬性oneInstance ，為了獲取這單例屬性，OneInstance 類提供了一個靜態方法getInstance()，它會返回oneInstance。

但是這個類既沒有構造體，也沒有屬性，所以要spring託管的話需要用到factory-method方法。

看spring xml配置檔案

<?xml version="1.0" encoding="UTF-8"?>
 

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context"
    xmlns:jdbc="http://www.springframework.org/schema/jdbc" xmlns:jee="http://www.springframework.org/schema/jee"
    xmlns:aop="http://www.springframework.org/schema/aop"
 
 xmlns:tx="http://www.springframework.org/schema/tx"
    xmlns:util="http://www.springframework.org/schema/util" xmlns:task="http://www.springframework.org/schema/task"
    xsi:schemaLocation="
        http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-4.0.xsd
        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-4.0.xsd
        http://www.springframework.org/schema/jdbc http://www.springframework.org/schema/jdbc/spring-jdbc-4.0.xsd
        http://www.springframework.org/schema/jee http://www.springframework.org/schema/jee/spring-jee-4.0.xsd
        http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-4.0.xsd
        http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util-4.0.xsd
        http://www.springframework.org/schema/task http://www.springframework.org/schema/task/spring-task-4.0.xsd
        http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-4.0.xsd"

    default-lazy-init="true">

    <description>Spring Configuration</description>

    <bean id="oneInstance" class="spring.ioc02.OneInstance"
    factory-method="getInstance"
    ></bean>




</beans>

spring初始化的時候會執行spring.ioc02.OneInstance裡的getInstance獲取例項，以完成bean的裝配。

再看一下單元測試：

/**
     * ioc02
     * <bean> 中使用factory-method屬性  完成沒建構函式情況下的例項化（單例）
     */
    @Test
    public void test3() {
        ApplicationContext context = new ClassPathXmlApplicationContext("ioc02.xml");

        OneInstance instance = (OneInstance) context.getBean("oneInstance");
        instance.finish();
    }

輸出結果是： 例項化完成