1. Overview

1.概述

In this quick article, we’ll use the Java 8 Stream API and the Introspector class – to invoke all getters found in a POJO.

在这篇快速文章中，我们将使用Java 8 Stream API和Introspector类–来调用在POJO中发现的所有getters。

We will create a stream of getters, inspect return values and see if a field value is null.

我们将创建一个获取器流，检查返回值并查看一个字段的值是否为null.。

2. Setup

2.设置

The only setup we need is to create a simple POJO class:

我们需要的唯一设置是创建一个简单的POJO类。

public class Customer {

    private Integer id;
    private String name;
    private String emailId;
    private Long phoneNumber;

    // standard getters and setters
}

3. Invoking Getter Methods

3.调用获取器方法

We’ll analyze the Customer class using Introspector; this provides an easy way for discovering properties, events, and methods supported by a target class.

我们将使用Introspector来分析Customer类；这为发现目标类所支持的属性、事件和方法提供了一种简单的方法。

We’ll first collect all the PropertyDescriptor instances of our Customer class. PropertyDescriptor captures all the info of a Java Bean property:

我们首先要收集我们的Customer类的所有PropertyDescriptor实例。PropertyDescriptor 捕获了一个 Java Bean 属性的所有信息。

PropertyDescriptor[] propDescArr = Introspector
  .getBeanInfo(Customer.class, Object.class)
  .getPropertyDescriptors();

Let’s now go over all PropertyDescriptor instances, and invoke the read method for every property:

现在让我们去看看所有的PropertyDescriptor实例，并为每个属性调用读取方法。

return Arrays.stream(propDescArr)
  .filter(nulls(customer))
  .map(PropertyDescriptor::getName)
  .collect(Collectors.toList());

The nulls predicate we use above checks if the property can be read invokes the getter and filters only null values:

我们在上面使用的nulls 谓词检查属性是否可以被读取，调用getter并只过滤空值。

private static Predicate<PropertyDescriptor> nulls(Customer customer) { 
    return = pd -> { 
        Method getterMethod = pd.getReadMethod(); 
        boolean result = false; 
        return (getterMethod != null && getterMethod.invoke(customer) == null); 
    }; 
}

Finally, let’s now create an instance of a Customer, set a few properties to null and test our implementation:

最后，让我们现在创建一个Customer的实例，将一些属性设置为null并测试我们的实现。

@Test
public void givenCustomer_whenAFieldIsNull_thenFieldNameInResult() {
    Customer customer = new Customer(1, "John", null, null);
	    
    List<String> result = Utils.getNullPropertiesList(customer);
    List<String> expectedFieldNames = Arrays
      .asList("emailId","phoneNumber");
	    
    assertTrue(result.size() == expectedFieldNames.size());
    assertTrue(result.containsAll(expectedFieldNames));      
}

4. Conclusion

4.结论

In this short tutorial, we made good use of the Java 8 Stream API and an Introspector instance – to invoke all getters and retrieve a list of null properties.

在这个简短的教程中，我们很好地利用了Java 8 Stream API和一个Introspector 实例–来调用所有的getters并检索一个空属性列表。

As usual, the code is available over on GitHub.

像往常一样，代码可以在GitHub上找到。