Spring—有参构造与无参构造

简述：
1.一般一个类他会自带一个默认的无参构造方法，如果你在其中写入了有参构造，在没有重新写入无参构造的情况下，该午餐构造方法就不存在了
2.在使用spring框架时，在使用一个类的时候，spring容器会自动对该类进行实例化，但是该类必须时无参构造的，相当于spring的默认设置
3.如果在使用spring框架时，实体类的初始化方法是有参构造，那么需要在配置文件中进行配置（即.xml文件）之后spring容器才能对该类进行实例化

具体实例如下：
1.首先看无参构造

1. 实体类

	package day02;
	
public class ExampleBean {
	private int age;
	private  double price;
	private float value;
	private String name;
	

	public int getAge() {
		return age;
	}
	public void setAge(int age) {
		this.age = age;
	}
	public double getPrice() {
		return price;
	}
	public void setPrice(double price) {
		this.price = price;
	}
	public float getValue() {
		return value;
	}
	public void setValue(float value) {
		this.value = value;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	@Override
	public String toString() {
		return "ExampleBean [age=" + age + ", price=" + price + ", value=" + value + ", name=" + name + "]";
	}
}

**

 2. 测试类

**

public class TestCase {
	ClassPathXmlApplicationContext ctx;
	@Before
	public void init() {
		ctx=new ClassPathXmlApplicationContext("applicationContext.xml");
	}


	@After
	public void destory() {
		ctx.close();
	}
	@Test
	public void ExampleBean() {
		ExampleBean bean=ctx.getBean("example",ExampleBean.class);
		System.out.println(bean.toString());
			}
	}
	

 3. 配置文件：applicationContext.xml文件（部分类容）

	
	<!-- 测试基本值的注入 -->
	<bean id="example" class="day02.ExampleBean">
	<property name="age" value="20"/>
	<property name="price" value="188.8"/>
	<property name="value" value="18.8"/>
	<property name="name" value="Tom"/>
	</bean> 

打印结果：
ExampleBean [age=20, price=188.8, value=18.8, name=Tom]

如果在ExampleBean中加入有参构造（部分代码）：

public ExampleBean(int age, double price, float value, String name) {
		this.age = age;
		this.price = price;
		this.value = value;
		this.name = name;
	}

那么配置文件红也需要改动：
如下：

<!-- 测试基本值的注入 -->
<!-- 这里的name对应需要被注入的参数名称,也就是构造函数的参数名称,value为需要被注入的值 -->
<bean id="example" class="day02.ExampleBean">
<constructor-arg name="age" value="20"></constructor-arg>
<constructor-arg name="price" value="188.8"></constructor-arg>
<constructor-arg name="value" value="18.8"></constructor-arg>
<constructor-arg name="name" value="Tom"></constructor-arg>

</bean> 

否则就会报如下错误：

警告: Exception encountered during context initialization - cancelling
refresh attempt:
org.springframework.beans.factory.BeanCreationException: Error
creating bean with name ‘example’ defined in class path resource
[applicationContext.xml]: Instantiation of bean failed; nested
exception is org.springframework.beans.BeanInstantiationException:
Failed to instantiate [day02.ExampleBean]: No default constructor
found; nested exception is java.lang.NoSuchMethodException:
day02.ExampleBean.()

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'example' defined in class path resource [applicationContext.xml]: Instantiation of bean failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [day02.ExampleBean]: No default constructor found; nested exception is java.lang.NoSuchMethodException: day02.ExampleBean.<init>()
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateBean(AbstractAutowireCapableBeanFactory.java:1155)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1099)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:513)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:483)
	at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:306)
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:761)
	at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:866)
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:542)
	at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:139)
	at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:83)
	at day02.TestCase.init(TestCase.java:12)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
	at java.lang.reflect.Method.invoke(Unknown Source)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
	at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:24)
	at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:27)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
	at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:86)
	at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:538)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:760)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:460)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:206)
Caused by: org.springframework.beans.BeanInstantiationException: Failed to instantiate [day02.ExampleBean]: No default constructor found; nested exception is java.lang.NoSuchMethodException: day02.ExampleBean.<init>()
	at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:85)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateBean(AbstractAutowireCapableBeanFactory.java:1147)
	... 37 more
Caused by: java.lang.NoSuchMethodException: day02.ExampleBean.<init>()
	at java.lang.Class.getConstructor0(Unknown Source)
	at java.lang.Class.getDeclaredConstructor(Unknown Source)
	at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:80)
	... 38 more

java.lang.NullPointerException
	at day02.TestCase.destory(TestCase.java:18)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
	at java.lang.reflect.Method.invoke(Unknown Source)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
	at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:33)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
	at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:86)
	at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:538)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:760)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:460)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:206)