私有构造函数不影响推土机

Question

为了理解 Dozer，我创建了两个 classes PrimaryType 和 SecondaryType 并尝试使用 Dozer 映射它们。 Dozer 能够无缝地映射它们。

尝试将 SecondaryType 的构造函数设为私有，但 Dozer 仍然能够成功创建 SecondaryType 类型的对象并能够映射其字段。谁能解释一下 Dozer 怎么可能实例化一个具有私有构造函数的 class？

PrimaryType.java

package in.yogi;

public class PrimaryType
{
    private String name;
    private int age;

    public PrimaryType(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "PrimaryType [name=" + name + ", age=" + age + "]";
    }

}

SecondaryType.java

package in.yogi;

public class SecondaryType
{
    private String name;
    private int age;
    private SecondaryType() {

    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    @Override
    public String toString() {
        return "SecondaryType [name=" + name + ", age=" + age + "]";
    }
}

AppMain.java

package in.yogi;

import org.dozer.DozerBeanMapper;
import org.dozer.Mapper;

import in.yogi.PrimaryType;

public class AppMain {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        System.out.println("This is the main Application!!!");
        PrimaryType sourceObject = new PrimaryType("Munsamy", 20);
        System.out.println(sourceObject);
        Mapper mapper = new DozerBeanMapper();
        SecondaryType destObject =  
            mapper.map(sourceObject, SecondaryType.class);
        System.out.println(destObject);
    }
}

输出：

PrimaryType [name=Munsamy, age=20]
log4j:WARN No appenders could be found for logger (org.dozer.config.GlobalSettings).
log4j:WARN Please initialize the log4j system properly.
log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.
SecondaryType [name=Munsamy, age=20]

Answer 1

这通常用 reflection. The Dozer documentation 解释：

The bean mapper is written in Java and relies heavily on the Jakarta Commons Bean Utils package for Java Bean utility methods.

阅读 Commons Beanutils 项目的文档，您可以找到所需的解释：

The Java language provides Reflection and Introspection APIs (see the java.lang.reflect and java.beans packages in the JDK Javadocs). However, these APIs can be quite complex to understand and utilize. The BeanUtils component provides easy-to-use wrappers around these capabilities.

你自己也可以轻松搞定：

public static void main(String[] args) throws Exception {
    Constructor<SecondaryType> constructor = SecondaryType.class.getDeclaredConstructor();
    constructor.setAccessible(true); // if not set accessible, you will get an exception
    SecondaryType instance = constructor.newInstance();
    System.out.println(instance);
}

私有构造函数不影响推土机

Private constructor not affecting Dozer

java

constructor

dozer

private-constructor