Spring---有参构造与无参构造
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
- 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）
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：
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.()
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.(ClassPathXmlApplicationContext.java:139)
at org.springframework.context.support.ClassPathXmlApplicationContext.(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.()
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.()
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)