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。
要在项目中使用SnakeYAML,需要添加Maven依赖项
- <dependency>
- <groupId>org.yaml</groupId>
- <artifactId>snakeyaml</artifactId>
- <version>1.25</version>
- </dependency>
-
该YAML
类是API的入口点:
- Yaml yaml = new Yaml()
-
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。
SnakeYAML
支持从String
或InputStream
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
-
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);
-
上面的代码生成以下输出:
- {firstName=John, lastName=Doe, age=20}
-
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。
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
- }
-
现在我么来加载:
- Yaml yaml = new Yaml();
- InputStream inputStream = this.getClass()
- .getClassLoader()
- .getResourceAsStream("customer.yaml");
- Customer customer = yaml.load(inputStream);
-
还有一种方法是使用Constructor:
- Yaml yaml = new Yaml(new Constructor(Customer.class));
-
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
-
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));
- }
-
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
-
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
- }
-
现在,我们来测试下Yaml
的load()
:
- @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());
- }
-
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}
-
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);
-
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
-
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);
- }
-
SnakeYAML
支持 将java对象序列化为yml。
我们将从一个将Map
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());
- }
-
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]
-
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());
- }
-
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);