使用Jackson将JSON数组转换为Java数组或列表


Convert JSON Array to a Java Array or List with Jackson

介绍

在本文中，我们将使用Jackson将JSON数组转换为Java Array和Java List。

由于我们正在使用Jackson，因此您必须将其添加到您的项目中。如果您使用的是Maven，则就像添加依赖项一样简单：

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<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    jackson-databind</artifactId>
    <version>2.11.2</version>
</dependency>

或者，如果您使用的是Gradle：

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compile group: 'com.fasterxml.jackson.core', name: 'jackson-databind', version: '2.11.2'

因为我们是从JSON映射到我们自己的对象，所以让我们继续定义POJO：

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public class Language {

    private String name;
    private String description;

    // Getters, setters and toString() method

从字符串读取JSON

让我们从读取String开始。字符串包含一系列编程语言，并带有简短说明：

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String json ="[{"name":"Java","description":"Java is a class-based, object-oriented programming Language."},{"name":"Python","description":"Python is an interpreted, high-level and general-purpose programming Language."}, {"name":"JS","description":"JS is a programming Language that conforms to the ECMAScript specification."}]";

使用Jackson的ObjectMapper类，可以很容易地读取值并将它们映射到一个对象或对象数组。我们只使用readValue()方法，传递JSON内容和我们想要映射到的类。由于我们要映射到Language的数组，因此我们还将在readValue()方法中指定它：

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// It's advised to use ObjectMapper as a singleton and reuse the instance
final ObjectMapper ObjectMapper = new ObjectMapper();
Language[] langs = ObjectMapper.readValue(json, Language[].class);

另外，您可以使用Jackson的TypeReference将值直接提取到列表中：

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List<Language> langList = ObjectMapper.readValue(json, new TypeReference<List<Language>>(){});

不建议使用TypeReference<>，您可以使用任何其他方法将数组转换为列表，例如：

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List<Language> langList = new ArrayList(Arrays.asList(langs));

然后打印出值：

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langList.forEach(x -> System.out.println(x.toString()));

结果是：

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Language{name='Java', description='Java is a class-based, object-oriented programming Language.'}
Language{name='Python', description='Python is an interpreted, high-level and general-purpose programming Language.'}
Language{name='JS', description='JS is a programming Language that conforms to the ECMAScript specification.'}

从文件读取JSON

我们并不总是处理String格式的JSON。通常，内容来自File。值得庆幸的是，Jackson使这项任务像上一个任务一样容易，我们只需将File提供给readValue()方法：

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final ObjectMapper ObjectMapper = new ObjectMapper();
List<Language> langList = ObjectMapper.readValue(
        new File("langs.json"),
        new TypeReference<List<Language>>(){});

langList.forEach(x -> System.out.println(x.toString()));

langs.json文件包含：

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[
  {
   "name":"Java",
   "description":"Java is a class-based, object-oriented programming Language that is designed to have as few implementation dependencies as possible."
  },
  {
   "name":"Python",
   "description":"Python is an interpreted, high-level and general-purpose programming Language. Created by Guido van Rossum and first released in 1991."
  },
  {
   "name":"JS",
   "description":"JS is a programming Language that conforms to the ECMAScript specification. JavaScript is high-level, often just-in-time compiled, and multi-paradigm."
  }
]

运行此代码将导致：

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Language{name='Java', description='Java is a class-based, object-oriented programming Language that is designed to have as few implementation dependencies as possible.'}
Language{name='Python', description='Python is an interpreted, high-level and general-purpose programming Language. Created by Guido van Rossum and first released in 1991.'}
Language{name='JS', description='JS is a programming Language that conforms to the ECMAScript specification. JavaScript is high-level, often just-in-time compiled, and multi-paradigm.'}

结论

在本文中，我们使用了Jackson来将JSON字符串和文件中的值解析并映射到Java数组和列表中。

通过readValue()方法，通过指定JSON内容(字符串或文件)并指定我们要映射到的POJO来完成此操作。