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Groovy里自定义JSON输出-JsonGenerator

白石

发布于 2019-08-23 02:00:01

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自定义JSON输出-JsonGenerator

Groovy 2.5.0增加了通过JsonGenerator实例自定义JSON输出。将对象转换为JSON字符串值的最简单方法是通过JsonOutput.toJson。此方法使用默认的JsonGenerator，其JSON输出具有合理的默认值。但是我们可以使用自定义生成器并创建JSON输出。要创建自定义生成器，我们使用可通过JsonGenerator.Options访问的构建器。通过流式的API，我们可以例如忽略输出中带有null值的字段，更改日期的日期格式，并按名称或值的类型忽略字段。我们可以通过将转换的实现添加为Closure或者实现JsonGenerator.Converter接口来为类型添加自定义转换器。要获取JSON字符串，我们只需调用生成器的toJson方法。

在下面的示例Groovy代码中，我们有一个带有数据的Map，我们想将它转换为JSON。首先我们使用默认生成器然后创建自己的生成器来自定义JSON输出：

// Sample class to be used in JSON.
@groovy.transform.TupleConstructor
class Student {
    String firstName, lastName
}
 
def data =
    [student: new Student('Hubert', 'Klein Ikkink'),
     dateOfBirth: Date.parse('yyyyMMdd', '19730709'),
     website: 'https://www.mrhaki.com'.toURL(),
     password: 'IamSecret',
     awake: Optional.empty(),
     married: Optional.of(true),
     location: null,
     currency: '\u20AC' /* Unicode EURO */]
      
 
import groovy.json.JsonGenerator
import groovy.json.JsonGenerator.Converter
         
// Default JSON generator. This generator is used by
// Groovy to create JSON if we don't specify our own.
// For this example we define the default generator
// explicitly to see the default output.      
def jsonDefaultOutput = new JsonGenerator.Options().build()
         
// Use generator to create JSON string.
def jsonDefaultResult = jsonDefaultOutput.toJson(data) // Or use JsonOutput.toJson(data)
 
assert jsonDefaultResult == '{"student":{"firstName":"Hubert","lastName":"Klein Ikkink"},' +
    '"dateOfBirth":"1973-07-08T23:00:00+0000","website":"https://www.mrhaki.com","password":"IamSecret",' +
    '"awake":{"present":false},"married":{"present":true},"location":null,"currency":"\\u20ac"}'
 
 
// Define custom rules for JSON that will be generated.
def jsonOutput =
    new JsonGenerator.Options()
        .excludeNulls()  // Do not include fields with value null.
        .dateFormat('EEEE dd-MM-yyyy', new Locale('nl', 'NL')) // Set format for dates.
        .timezone('Europe/Amsterdam') // Set timezone to be used for formatting dates.
        .excludeFieldsByName('password')  // Exclude fields with given name(s).
        .excludeFieldsByType(URL)  // Exclude fields of given type(s).
        .disableUnicodeEscaping()  // Do not escape UNICODE.
        .addConverter(Optional) { value -> value.orElse('UNKNOWN') } // Custom converter for given type defined as Closure.
        .addConverter(new Converter() {  // Custom converter implemented via Converter interface.
         
            /**
             * Indicate which type this converter can handle.
             */
            boolean handles(Class<?> type) {
                return Student.isAssignableFrom(type)
            }
             
            /**
             * Logic to convert Student object.
             */
            Object convert(Object student, String key) {
                "$student.firstName $student.lastName"
            }
             
        })
        .build()  // Create the converter instance.
 
// Use generator to create JSON from Map data structure.
def jsonResult = jsonOutput.toJson(data)
 
assert jsonResult == '{"student":"Hubert Klein Ikkink",' +
    '"dateOfBirth":"maandag 09-07-1973",' +
    '"awake":"UNKNOWN","married":true,"currency":"€"}'

JsonBuilder和StreamingJsonBuilder类现在也支持使用JsonGenerator实例。需要创建JSON输出时使用生成器。使用自定义生成器不会更改构建器的内部数据结构。

在下面的示例中，我们使用前一个示例的自定义生成器，并将其应用于JsonBuilder和StreamingJsonBuilder实例：

import groovy.json.JsonBuilder
 
// We can use a generator instance as constructor argument
// for JsonBuilder. The generator is used when we create the
// JSON string. It will not effecct the internal JSON data structure.
def jsonBuilder = new JsonBuilder(jsonOutput)
jsonBuilder {
    student new Student('Hubert', 'Klein Ikkink')
    dateOfBirth Date.parse('yyyyMMdd', '19730709')
    website 'https://www.mrhaki.com'.toURL()
    password 'IamSecret'
    awake Optional.empty()
    married Optional.of(true)
    location null
    currency  '\u20AC'
}
 
def jsonBuilderResult = jsonBuilder.toString()
 
assert jsonBuilderResult == '{"student":"Hubert Klein Ikkink",' +
    '"dateOfBirth":"maandag 09-07-1973",' +
    '"awake":"UNKNOWN","married":true,"currency":"€"}'
 
// The internal structure is unaffected by the generator.
assert jsonBuilder.content.password == 'IamSecret'
assert jsonBuilder.content.website.host == 'www.mrhaki.com'
 
 
import groovy.json.StreamingJsonBuilder
 
new StringWriter().withWriter { output ->
 
    // As with JsonBuilder we can provide a custom generator via
    // the constructor for StreamingJsonBuilder.
    def jsonStreamingBuilder = new StreamingJsonBuilder(output, jsonOutput)
    jsonStreamingBuilder {
        student new Student('Hubert', 'Klein Ikkink')
        dateOfBirth Date.parse('yyyyMMdd', '19730709')
        website 'https://www.mrhaki.com'.toURL()
        password 'IamSecret'
        awake Optional.empty()
        married Optional.of(true)
        location null
        currency  '\u20AC'
    }
 
    def jsonStreamingBuilderResult = output.toString()
     
    assert jsonStreamingBuilderResult == '{"student":"Hubert Klein Ikkink",' +
        '"dateOfBirth":"maandag 09-07-1973",' +
        '"awake":"UNKNOWN","married":true,"currency":"€"}'
}