什么是Pojo Class

1. Overview

In this short tutorial, we’ll investigate the definition of “Plain Old Java Object” or POJO for short.

We’ll look at how a POJO compares to a JavaBean, and how turning our POJOs into JavaBeans can be helpful.

2. Plain Old Java Objects

2.1. What Is a POJO?

When we talk about a POJO, what we’re describing is a straightforward type with no references to any particular frameworks. A POJO has no naming convention for our properties and methods.

Let’s create a basic employee POJO. It’ll have three properties; first name, last name, and start date:

public class EmployeePojo {

    public String firstName;
    public String lastName;
    private LocalDate startDate;

    public EmployeePojo(String firstName, String lastName, LocalDate startDate) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.startDate = startDate;
    }

    public String name() {
        return this.firstName + " " + this.lastName;
    }

    public LocalDate getStart() {
        return this.startDate;
    }
}

This class can be used by any Java program as it’s not tied to any framework.

But, we aren’t following any real convention for constructing, accessing, or modifying the class’s state.

This lack of convention causes two problems:

First, it increases the learning curve for coders trying to understand how to use it.

Second, it may limit a framework’s ability to favor convention over configuration, understand how to use the class, and augment its functionality.

To explore this second point, let’s work with EmployeePojo using reflection. Thus, we’ll start to find some of its limitations.

2.2. Reflection with a POJO

Let’s add the commons-beanutils dependency to our project:

<dependency>
    <groupId>commons-beanutils</groupId>
    <artifactId>commons-beanutils</artifactId>
    <version>1.9.4</version>
</dependency>

And now, let’s inspect the properties of our POJO:

List<String> propertyNames =
  PropertyUtils.getPropertyDescriptors(EmployeePojo.class).stream()
    .map(PropertyDescriptor::getDisplayName)
    .collect(Collectors.toList());

If we were to print out propertyNames to the console, we’d only see:

[start]

Here, we see that we only get start as a property of the class. PropertyUtils failed to find the other two.

We’d see the same kind of outcome were we to use other libraries like Jackson to process EmployeePojo.

Ideally, we’d see all our properties: firstName, lastName, and startDate. And the good news is that many Java libraries support by default something called the JavaBean naming convention.

3. JavaBeans

3.1. What Is a JavaBean?

A JavaBean is still a POJO but introduces a strict set of rules around how we implement it:

• Access levels – our properties are private and we expose getters and setters
• Method names – our getters and setters follow the getX and setX convention (in the case of a boolean, isX can be used for a getter)
• Default Constructor – a no-argument constructor must be present so an instance can be created without providing arguments, for example during deserialization
• Serializable – implementing the Serializable interface allows us to store the state

3.2. EmployeePojo as a JavaBean

So, let’s try converting EmployeePojo into a JavaBean:

public class EmployeeBean implements Serializable {

    private static final long serialVersionUID = -3760445487636086034L;
    private String firstName;
    private String lastName;
    private LocalDate startDate;

    public EmployeeBean() {
    }

    public EmployeeBean(String firstName, String lastName, LocalDate startDate) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.startDate = startDate;
    }

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    //  additional getters/setters

}

3.3. Reflection with a JavaBean

When we inspect our bean with reflection, now we get the full list of the properties:

[firstName, lastName, startDate]

4. Tradeoffs When Using JavaBeans

So, we’ve shown a way in which JavaBeans are helpful. Keep in mind that every design choice comes with tradeoffs.

When we use JavaBeans we should also be mindful of some potential disadvantages:

• Mutability – our JavaBeans are mutable due to their setter methods – this could lead to concurrency or consistency issues
• Boilerplate – we must introduce getters for all properties and setters for most, much of this might be unnecessary
• Zero-argument Constructor – we often need arguments in our constructors to ensure the object gets instantiated in a valid state, but the JavaBean standard requires us to provide a zero-argument constructor

Given these tradeoffs, frameworks have also adapted to other bean conventions over the years.

5. Conclusion

In this tutorial, we compared POJOs with JavaBeans.

First, we learned a POJO is a Java object that is bound to no specific framework, and that a JavaBean is a special type of POJO with a strict set of conventions.

Then, we saw how some frameworks and libraries harness the JavaBean naming convention to discover a class’s properties.

As usual, the examples are available over on GitHub.