在使用Ajax: Hibernate Entity => json, Flex RemoteObject: Hibernate Entity => ActionScript Object的過程,經常碰到如下問題:
問題:
1.Hibernate碰到延遲加載的屬性訪問時如果session被關閉則拋出LazyInitializationException
2.Hibernate中的one-to-many等關聯關系在序列化時如果沒有控制,則將整個數據庫都有可能被全部序列化
3.過多的使用DTO/ValueObject解決這個問題.
解決辦法:
對Entity對象生成一個動態代理,攔截getXXXX()方法,如果訪問的是延遲加載的屬性,則return null,而不拋出LazyInitializationException,遞歸生成屬性的代碼,只要碰到未延遲加載的屬性,而序列化會自動停止.避免將整個Entity序列化傳播,導致可能序列化整個數據庫的問題.
類似解決方案:
dwr中HibernateConverter,在序列化時如上類似,碰到延遲加載的屬性自動停止convert工作.而HibernateBeanSerializer則一勞永逸,無論object => json都可以工作.
使用用例:
Java代碼
//role為原始對象
role = (Role)roleDao.getById(new Long(1));
//生成的動態代理proxyRole,訪問延遲加載的屬性將return null
Role proxyRole = (Role)new HibernateBeanSerializer<Role>(role).getProxy();
assertNotNull(role.getResource());
assertNull(proxyRole.getResource()); //延遲加載,為null
Hibernate.initialize(role.getResource()); //抓取進來
assertNotNull(proxyRole.getResource()); //不為null
源碼.
Java代碼
package cn.org.rapid_framework.util;
import java.lang.reflect.Modifier;
import java.util.ArrayList;
import java.util.Collection;
import java.util.LinkedHashMap;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import org.aopalliance.intercept.MethodInterceptor;
import org.aopalliance.intercept.MethodInvocation;
import org.hibernate.Hibernate;
import org.hibernate.collection.PersistentCollection;
import org.hibernate.proxy.HibernateProxy;
import org.springframework.aop.framework.ProxyFactory;
import org.springframework.util.StringUtils;
/**
* 用於Hibernate Object 的序列化,訪問延遲加載的屬性不會拋出LazyInitializationException,而會返回null值.
* 使用:
* <pre>
* Blog proxyBlog = new HibernateBeanSerializer(blog).getProxy();
* </pre>
* @author badqiu
* @param <T>
*/
public class HibernateBeanSerializer <T> {
T proxy = null;
/**
*/
public HibernateBeanSerializer(T object,String... excludesProperties) {
if(object == null) {
this.proxy = null;
}else {
ProxyFactory pf = new ProxyFactory();
pf.setTargetClass(object.getClass());
pf.setOptimize(true);
pf.setTarget(object);
pf.setProxyTargetClass(true);
pf.setOpaque(true);
pf.setExposeProxy(true);
pf.setPreFiltered(true);
HibernateBeanSerializerAdvice beanSerializerAdvice = new HibernateBeanSerializerAdvice();
beanSerializerAdvice.setExcludesProperties(excludesProperties);
pf.addAdvice(beanSerializerAdvice);
this.proxy = (T)pf.getProxy();
}
}
public T getProxy(){
return this.proxy;
}
static private class HibernateBeanSerializerAdvice implements MethodInterceptor {
private String[] excludesProperties = new String[0];
public String[] getExcludesProperties() {
return excludesProperties;
}
public void setExcludesProperties(String[] excludesProperties) {
this.excludesProperties = excludesProperties == null ? new String[0] : excludesProperties;
}
public Object invoke(MethodInvocation mi) throws Throwable {
String propertyName = getPropertyName(mi.getMethod().getName());
Class returnType = mi.getMethod().getReturnType();
if(propertyName == null) {
return mi.proceed();
}
if(!Hibernate.isPropertyInitialized(mi.getThis(), propertyName)) {
return null;
}
if(isExclude(mi, propertyName)) {
return null;
}
Object returnValue = mi.proceed();
return processReturnValue(returnType, returnValue);
}
private Object processReturnValue(Class returnType, Object returnValue) {
if(returnValue == null)
return null;
if(returnType != null && Modifier.isFinal(returnType.getModifiers())) {
return returnValue;
}
//This might be a lazy-collection so we need to double check
if(!Hibernate.isInitialized(returnValue)) {
return null;
}
//this is Hibernate Object
if(returnValue instanceof HibernateProxy) {
return new HibernateBeanSerializer(returnValue).getProxy();
}else if(returnValue instanceof PersistentCollection) {
if(returnType.isAssignableFrom(Map.class)) {
Map proxyMap = new LinkedHashMap();
Map map = (Map)returnValue;
Set<Map.Entry> entrySet = map.entrySet();
for(Map.Entry entry : entrySet) {
proxyMap.put(entry.getKey(), new HibernateBeanSerializer(entry.getValue()));
}
return proxyMap;
}
Collection proxyCollection = null;
if(returnType.isAssignableFrom(Set.class)) {
proxyCollection = new LinkedHashSet();
}else if(returnType.isAssignableFrom(List.class)) {
proxyCollection = new ArrayList();
}else {
return returnValue;
}
for(Object o : (Collection)returnValue) {
proxyCollection.add(new HibernateBeanSerializer(o).getProxy());
}
return proxyCollection;
}else {
return returnValue;
}
}
private boolean isExclude(MethodInvocation mi, String propertyName)
throws Throwable {
for(String excludePropertyName : excludesProperties) {
if(propertyName.equals(excludePropertyName)) {
return true;
}
}
return false;
}
private static String getPropertyName(String methodName) {
String propertyName = null;
if(methodName.startsWith("get")) {
propertyName = methodName.substring("get".length());
}else if(methodName.startsWith("is")) {
propertyName = methodName.substring("is".length());
}else if(methodName.startsWith("set")) {
propertyName = methodName.substring("set".length());
}
return propertyName == null ? null : StringUtils.uncapitalize(propertyName);
}
}
}
另這個類屬於rapid-framework的一部分,v2.0版本的flex RemoteObject將采用這個辦法.preview版本即將發布
