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JAVA使用SnakeYAML解析与序列化YAML

章越
2023-12-01

1.概述

本文,我们将学习如何使用SnakeYAML库将
YAML文档转换为Java对象,以及JAVA对象如何序列化为YAML文档

2.项目设置

要在项目中使用SnakeYAML,需要添加Maven依赖项(可在此处找到最新版本):

<dependency>
    <groupId>org.yaml</groupId>
    <artifactId>snakeyaml</artifactId>
    <version>1.25</version>
</dependency>
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3.入口点

YAML类是API的入口点:

Yaml yaml = new Yaml()
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由于实现不是线程安全的,因此不同的线程必须具有自己的Yaml实例。

4.加载YAML文档

SnakeYAML支持从StringInputStream加载文档,我们从定义一个简单的YAML文档开始,然后将文件命名为customer.yaml

firstName: "John"
lastName: "Doe"
age: 20
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4.1 基本用法

现在,我们将使用Yaml类来解析上述YAML文档:

Yaml yaml = new Yaml();
InputStream inputStream = this.getClass()
  .getClassLoader()
  .getResourceAsStream("customer.yaml");
Map<String, Object> obj = yaml.load(inputStream);
System.out.println(obj);
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上面的代码生成以下输出:

{firstName=John, lastName=Doe, age=20}
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默认情况下,load()方法返回一个Map对象。查询Map对象时,我们需要事先知道属性键的名称,否则容易出错。更好的办法是自定义类型。

4.2 自定义类型解析

SnakeYAML提供了一种将文档解析为自定义类型的方法

让我们定义一个Customer类,然后尝试再次加载该文档:

public class Customer {
 
    private String firstName;
    private String lastName;
    private int age;
 
    // getters and setters
}
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现在我么来加载:

Yaml yaml = new Yaml();
InputStream inputStream = this.getClass()
 .getClassLoader()
 .getResourceAsStream("customer.yaml");
Customer customer = yaml.load(inputStream);
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还有一种方法是使用Constructor:

Yaml yaml = new Yaml(new Constructor(Customer.class));
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4.3 隐式类型

如果没有为给定属性定义类型,则库会自动将值转换为隐式type

例如:

1.0 -> Float
42 -> Integer
2009-03-30 -> Date
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让我们使用一个TestCase来测试这种隐式类型转换:

@Test
public void whenLoadYAML_thenLoadCorrectImplicitTypes() {
   Yaml yaml = new Yaml();
   Map<Object, Object> document = yaml.load("3.0: 2022-8-17");
  
   assertNotNull(document);
   assertEquals(1, document.size());
   assertTrue(document.containsKey(3.0d));   
}
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4.4 嵌套对象

SnakeYAML 支持嵌套的复杂类型。

让我们向“ customer.yaml”添加“ 联系方式”  和“ 地址” 详细信息并将新文件另存为customer_with_contact_details_and_address.yaml.

现在,我们将分析新的YAML文档:

firstName: "John"
lastName: "Doe"
age: 31
contactDetails:
   - type: "mobile"
     number: 123456789
   - type: "landline"
     number: 456786868
homeAddress:
   line: "Xyz, DEF Street"
   city: "City Y"
   state: "State Y"
   zip: 345657
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我们来更新java类:

public class Customer {
    private String firstName;
    private String lastName;
    private int age;
    private List<Contact> contactDetails;
    private Address homeAddress;    
    // getters and setters
}

public class Contact {
    private String type;
    private int number;
    // getters and setters
}

public class Address {
    private String line;
    private String city;
    private String state;
    private Integer zip;
    // getters and setters
}
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现在,我们来测试下Yamlload()

@Test
public void
  whenLoadYAMLDocumentWithTopLevelClass_thenLoadCorrectJavaObjectWithNestedObjects() {
  
    Yaml yaml = new Yaml(new Constructor(Customer.class));
    InputStream inputStream = this.getClass()
      .getClassLoader()
      .getResourceAsStream("yaml/customer_with_contact_details_and_address.yaml");
    Customer customer = yaml.load(inputStream);
  
    assertNotNull(customer);
    assertEquals("John", customer.getFirstName());
    assertEquals("Doe", customer.getLastName());
    assertEquals(31, customer.getAge());
    assertNotNull(customer.getContactDetails());
    assertEquals(2, customer.getContactDetails().size());
     
    assertEquals("mobile", customer.getContactDetails()
      .get(0)
      .getType());
    assertEquals(123456789, customer.getContactDetails()
      .get(0)
      .getNumber());
    assertEquals("landline", customer.getContactDetails()
      .get(1)
      .getType());
    assertEquals(456786868, customer.getContactDetails()
      .get(1)
      .getNumber());
    assertNotNull(customer.getHomeAddress());
    assertEquals("Xyz, DEF Street", customer.getHomeAddress()
      .getLine());
}
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4.5 类型安全的集合

当给定Java类的一个或多个属性是泛型集合类时,需要通过TypeDescription来指定泛型类型,以以便可以正确解析。

让我们假设一个 一个Customer拥有多个Contact

firstName: "John"
lastName: "Doe"
age: 31
contactDetails:
   - { type: "mobile", number: 123456789}
   - { type: "landline", number: 123456789}
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为了能正确解析,我们可以在顶级类上为给定属性指定TypeDescription

Constructor constructor = new Constructor(Customer.class);
TypeDescription customTypeDescription = new TypeDescription(Customer.class);
customTypeDescription.addPropertyParameters("contactDetails", Contact.class);
constructor.addTypeDescription(customTypeDescription);
Yaml yaml = new Yaml(constructor);
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4.6 载入多个文件

在某些情况下,单个文件中可能有多个YAML文档,而我们想解析所有文档。所述YAML类提供了一个LOADALL()方法来完成这种类型的解析。

假设下面的内容在一个文件中:

---
firstName: "John"
lastName: "Doe"
age: 20
---
firstName: "Jack"
lastName: "Jones"
age: 25
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我们可以使用loadAll()方法解析以上内容,如以下代码示例所示:

@Test
public void whenLoadMultipleYAMLDocuments_thenLoadCorrectJavaObjects() {
    Yaml yaml = new Yaml(new Constructor(Customer.class));
    InputStream inputStream = this.getClass()
      .getClassLoader()
      .getResourceAsStream("yaml/customers.yaml");
 
    int count = 0;
    for (Object object : yaml.loadAll(inputStream)) {
        count++;
        assertTrue(object instanceof Customer);
    }
    assertEquals(2,count);
}
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5.生成YAML文件

SnakeYAML 支持 将java对象序列化为yml。

5.1 基本用法

我们将从一个将Map <String,Object>的实例转储到YAML文档(String)的简单示例开始:

@Test
public void whenDumpMap_thenGenerateCorrectYAML() {
    Map<String, Object> data = new LinkedHashMap<String, Object>();
    data.put("name", "Silenthand Olleander");
    data.put("race", "Human");
    data.put("traits", new String[] { "ONE_HAND", "ONE_EYE" });
    Yaml yaml = new Yaml();
    StringWriter writer = new StringWriter();
    yaml.dump(data, writer);
    String expectedYaml = "name: Silenthand Olleander\nrace: Human\ntraits: [ONE_HAND, ONE_EYE]\n";
 
    assertEquals(expectedYaml, writer.toString());
}
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上面的代码产生以下输出(请注意,使用LinkedHashMap的实例将保留输出数据的顺序):

name: Silenthand Olleander
race: Human
traits: [ONE_HAND, ONE_EYE]
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5.2 自定义Java对象

我们还可以选择将自定义Java类型转储到输出流中

@Test
public void whenDumpACustomType_thenGenerateCorrectYAML() {
    Customer customer = new Customer();
    customer.setAge(45);
    customer.setFirstName("Greg");
    customer.setLastName("McDowell");
    Yaml yaml = new Yaml();
    StringWriter writer = new StringWriter();
    yaml.dump(customer, writer);        
    String expectedYaml = "!!com.baeldung.snakeyaml.Customer {age: 45, contactDetails: null, firstName: Greg,\n  homeAddress: null, lastName: McDowell}\n";
 
    assertEquals(expectedYaml, writer.toString());
}
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生成内容会包含!!com.baeldung.snakeyaml.Customer,为了避免在输出文件中使用标签名,我们可以使用库提供的  dumpAs()方法。

因此,在上面的代码中,我们可以进行以下调整以删除标记:

yaml.dumpAs(customer, Tag.MAP, null);

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