java反射(Field的应用)

//$Id: DirectPropertyAccessor.java 11405 2007-04-15 12:50:34Z max.andersen@jboss.com $
package org.hibernate.property;

import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.util.Map;

import org.hibernate.HibernateException;
import org.hibernate.PropertyAccessException;
import org.hibernate.PropertyNotFoundException;
import org.hibernate.engine.SessionFactoryImplementor;
import org.hibernate.engine.SessionImplementor;
import org.hibernate.util.ReflectHelper;

/**
 * Accesses fields directly.
 * @author Gavin King
 */
public class DirectPropertyAccessor implements PropertyAccessor {

    public static final class DirectGetter implements Getter {
        private final transient Field field;
        private final Class clazz;
        private final String name;
        DirectGetter(Field field, Class clazz, String name) {
            this.field = field;
            this.clazz = clazz;
            this.name = name;
        }
        public Object get(Object target) throws HibernateException {
            try {
                return field.get(target);//返回指定对象上此 Field 表示的字段的值。
            }
            catch (Exception e) {
                throw new PropertyAccessException(e, "could not get a field value by reflection", false, clazz, name);
            }
        }

        public Object getForInsert(Object target, Map mergeMap, SessionImplementor session) {
            return get( target );
        }

        public Method getMethod() {
            return null;
        }
        public String getMethodName() {
            return null;
        }
        public Class getReturnType() {
            return field.getType();
        }

        Object readResolve() {
            return new DirectGetter( getField(clazz, name), clazz, name );
        }
        
        public String toString() {
            return "DirectGetter(" + clazz.getName() + '.' + name + ')';
        }
    }

    public static final class DirectSetter implements Setter {
        private final transient Field field;
        private final Class clazz;
        private final String name;
        DirectSetter(Field field, Class clazz, String name) {
            this.field = field;
            this.clazz = clazz;
            this.name = name;
        }
        public Method getMethod() {
            return null;
        }
        public String getMethodName() {
            return null;
        }
        public void set(Object target, Object value, SessionFactoryImplementor factory) throws HibernateException {
            try {
                field.set(target, value);//将指定对象变量上此 Field 对象表示的字段设置为指定的新值。
            }
            catch (Exception e) {
                if(value == null && field.getType().isPrimitive()) {
                    throw new PropertyAccessException(
                            e, 
                            "Null value was assigned to a property of primitive type", 
                            true, 
                            clazz, 
                            name
                        );                    
                } else {
                    throw new PropertyAccessException(e, "could not set a field value by reflection", true, clazz, name);
                }
            }
        }

        public String toString() {
            return "DirectSetter(" + clazz.getName() + '.' + name + ')';
        }
        
        Object readResolve() {
            return new DirectSetter( getField(clazz, name), clazz, name );
        }
    }

    private static Field getField(Class clazz, String name) throws PropertyNotFoundException {
        if ( clazz==null || clazz==Object.class ) {
            throw new PropertyNotFoundException("field not found: " + name); 
        }
        Field field;
        try {
            field = clazz.getDeclaredField(name);//返回一个 Field 对象，该对象反映此 Class 对象所表示的类或接口的指定已声明字段。
        }
        catch (NoSuchFieldException nsfe) {
            field = getField( clazz, clazz.getSuperclass(), name );
        }
        if ( !ReflectHelper.isPublic(clazz, field) ) field.setAccessible(true);
        return field;
    }

    private static Field getField(Class root, Class clazz, String name) throws PropertyNotFoundException {
        if ( clazz==null || clazz==Object.class ) {
            throw new PropertyNotFoundException("field [" + name + "] not found on " + root.getName()); 
        }
        Field field;
        try {
            field = clazz.getDeclaredField(name);
        }
        catch (NoSuchFieldException nsfe) {
            field = getField( root, clazz.getSuperclass(), name );
        }
        if ( !ReflectHelper.isPublic(clazz, field) ) field.setAccessible(true);
        return field;
    }
    
    public Getter getGetter(Class theClass, String propertyName)
        throws PropertyNotFoundException {
        return new DirectGetter( getField(theClass, propertyName), theClass, propertyName );
    }

    public Setter getSetter(Class theClass, String propertyName)
        throws PropertyNotFoundException {
        return new DirectSetter( getField(theClass, propertyName), theClass, propertyName );
    }

}