Hibernate实体为什么要提供一个无参的构造函数
@Entity
@Table(name="user")
public class Person {

private int ID;
private String name;
private String cellphone;
/*
* 为什么需要一个默认的构造函数
*/
/*public Person() {
}*/

public Person(String name, String cellphone) {
this.name = name;
this.cellphone = cellphone;
}

@Id
public int getID() {
return ID;
}

public void setID(int iD) {
ID = iD;
}

@Column(name = "NAME")
public String getName() {
return name;
}

public void setName(String name) {
this.name = name;
}

@Column(name = "CELLPHONE")
public String getCellphone() {
return cellphone;
}

public void setCellphone(String cellphone) {
this.cellphone = cellphone;
}

}
如上所示把无参构造函数注释调以后，运行测试用例
@Test
public void testUpdate() {
Person person = personService.getPerson(1);
//....
person.setName("wang");
personService.update(person);
}
报如下异常：
org.hibernate.InstantiationException: No default constructor for entity: com.gosophia.entity.Person
at org.hibernate.tuple.PojoInstantiator.instantiate(PojoInstantiator.java:107)
at org.hibernate.tuple.PojoInstantiator.instantiate(PojoInstantiator.java:123)
at org.hibernate.tuple.entity.AbstractEntityTuplizer.instantiate(AbstractEntityTuplizer.java:374)
at org.hibernate.persister.entity.AbstractEntityPersister.instantiate(AbstractEntityPersister.java:3634)
at org.hibernate.impl.SessionImpl.instantiate(SessionImpl.java:1302)
at org.hibernate.impl.SessionImpl.instantiate(SessionImpl.java:1291)
at org.hibernate.loader.Loader.instanceNotYetLoaded(Loader.java:1323)
at org.hibernate.loader.Loader.getRow(Loader.java:1230)
at org.hibernate.loader.Loader.getRowFromResultSet(Loader.java:603)
at org.hibernate.loader.Loader.doQuery(Loader.java:724)
at org.hibernate.loader.Loader.doQueryAndInitializeNonLazyCollections(Loader.java:259)
at org.hibernate.loader.Loader.loadEntity(Loader.java:1881)
at org.hibernate.loader.entity.AbstractEntityLoader.load(AbstractEntityLoader.java:71)
at org.hibernate.loader.entity.AbstractEntityLoader.load(AbstractEntityLoader.java:65)
at org.hibernate.persister.entity.AbstractEntityPersister.load(AbstractEntityPersister.java:3072)
at org.hibernate.event.def.DefaultLoadEventListener.loadFromDatasource(DefaultLoadEventListener.java:434)
at org.hibernate.event.def.DefaultLoadEventListener.doLoad(DefaultLoadEventListener.java:415)
at org.hibernate.event.def.DefaultLoadEventListener.load(DefaultLoadEventListener.java:165)
at org.hibernate.event.def.DefaultLoadEventListener.proxyOrLoad(DefaultLoadEventListener.java:223)
at org.hibernate.event.def.DefaultLoadEventListener.onLoad(DefaultLoadEventListener.java:126)
at org.hibernate.impl.SessionImpl.fireLoad(SessionImpl.java:905)
at org.hibernate.impl.SessionImpl.get(SessionImpl.java:842)
at org.hibernate.impl.SessionImpl.get(SessionImpl.java:835)
at com.gosophia.service.impl.PersonServiceBean.getPerson(PersonServiceBean.java:27)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:307)
at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:182)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:149)

at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:106)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:171)
at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:204)
at $Proxy12.getPerson(Unknown Source)
at PersonServiceTest.testUpdate(PersonServiceTest.java:33)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:44)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:41)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)
at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:28)
at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:31)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:73)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:46)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:180)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:41)
at org.junit.runners.ParentRunner$1.evaluate(ParentRunner.java:173)
at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:28)
at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:31)
at org.junit.runners.ParentRunner.run(ParentRunner.java:220)
at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:46)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)
哪为高手可以分析一下Hibernate为什么要实体提供一个无参的构造函数......

[解决办法]
Hibernate进行反转的时候需要这个无参的构造函数作为支持
因为必须要有这个东西
否则Hibernate无法进行反转
如此而已

若要生成类中的实例的话
Hibernate根本不需要知道你里面还有什么参数和属性
所以用无参的来构造是最简便的

比如说
你Class里有name的属性
可是Student里没有name的属性
这样在创建的时候自然不好统一协调啦
于是就需要无参的构造函数
[解决办法]
System.out.print(Class.forName("java.lang.String").newInstance().getClass().getName());
这个需要无参构造函数
[解决办法]
当查询的时候返回的实体类是一个对象实例，是hibernate动态通过反射生成的
反射的Class.forName("className").newInstance();需要对应的类提供一个无参构造函数