Field类中包含了字段定义的详细信息，可以使用Field类中提供的方法来获取字段的信息，下面我们先获取Field对象，再介绍如何使用Field类中提供的方法。

一、准备工作

在src/test/java目录的cn.horse.reflect.entity包下新建BaseEntity类、UserInfoEntity类；新建EntityAnnotation、InheritedEntityAnnotation、UserInfoEntityAnnotation注解

BaseEntity类：

package cn.horse.reflect.entity;@EntityAnnotation
@InheritedEntityAnnotation
public abstract class BaseEntity {@EntityAnnotation@InheritedEntityAnnotationpublic Long id;@InheritedEntityAnnotationpublic BaseEntity() { }@EntityAnnotation@InheritedEntityAnnotationpublic BaseEntity(Long id) {this.id = id;}@EntityAnnotation@InheritedEntityAnnotationpublic void setId(Long id) {this.id = id;}public Long getId() {return id;}
}

UserInfoEntity类：

package cn.horse.reflect.entity;@UserInfoEntityAnnotation
public class UserInfoEntity extends BaseEntity {@UserInfoEntityAnnotationpublic String name;@UserInfoEntityAnnotationprivate Integer age;@UserInfoEntityAnnotationprivate UserInfoEntity() {}@UserInfoEntityAnnotationpublic UserInfoEntity(Long id) {super(id);}@UserInfoEntityAnnotationpublic UserInfoEntity(Long id, String name, Integer age) {super(id);this.name = name;this.age = age;}public void setAge(Integer age) {this.age = age;}public Integer getAge() {return age;}private String getName() {return name;}private void check() { }@UserInfoEntityAnnotation@Overridepublic void setId(Long id) {super.setId(id);}
}

EntityAnnotation注解：

package cn.horse.reflect.entity;import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;import static java.lang.annotation.ElementType.*;@Target({TYPE, FIELD, METHOD, PARAMETER, CONSTRUCTOR, LOCAL_VARIABLE})
@Retention(RetentionPolicy.RUNTIME)
public @interface EntityAnnotation {
}

InheritedEntityAnnotation注解：

package cn.horse.reflect.entity;import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;import static java.lang.annotation.ElementType.*;
import static java.lang.annotation.ElementType.LOCAL_VARIABLE;@Target({TYPE, FIELD, METHOD, PARAMETER, CONSTRUCTOR, LOCAL_VARIABLE})
@Retention(RetentionPolicy.RUNTIME)
@Inherited
public @interface InheritedEntityAnnotation {
}

UserInfoEntityAnnotation注解：

package cn.horse.reflect.entity;import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;import static java.lang.annotation.ElementType.*;
import static java.lang.annotation.ElementType.LOCAL_VARIABLE;@Target({TYPE, FIELD, METHOD, PARAMETER, CONSTRUCTOR, LOCAL_VARIABLE})
@Retention(RetentionPolicy.RUNTIME)
public @interface UserInfoEntityAnnotation {
}

二、获取Field对象

在src/test/java目录的cn.horse.reflect包下新建GetFieldInstanceTests测试类

GetFieldInstanceTests类：

package cn.horse.reflect;import cn.horse.reflect.entity.UserInfoEntity;
import org.junit.jupiter.api.Test;import java.lang.reflect.Field;
import java.util.Arrays;
import java.util.Collection;
import java.util.List;
import java.util.stream.Collectors;import static org.assertj.core.api.Assertions.assertThat;
import static org.assertj.core.api.Assertions.assertThatExceptionOfType;public class GetFieldInstanceTests {static final Class<UserInfoEntity> USER_INFO_ENTITY_CLASS = UserInfoEntity.class;@Testpublic void testGetFields() {// getFields 获取当前类及其父类中使用public定义的字段Collection<String> fieldNameList = Arrays.stream(USER_INFO_ENTITY_CLASS.getFields()).map(Field::getName).collect(Collectors.toList());List<String> expectedFieldNameList = Arrays.asList("name", "id");assertThat(fieldNameList.size()).isEqualTo(expectedFieldNameList.size());assertThat(fieldNameList).containsAll(expectedFieldNameList);}@Testpublic void testGetField() throws NoSuchFieldException {assertThat(USER_INFO_ENTITY_CLASS.getField("id")).isNotNull();assertThat(USER_INFO_ENTITY_CLASS.getField("name")).isNotNull();assertThatExceptionOfType(NoSuchFieldException.class).isThrownBy(() -> { USER_INFO_ENTITY_CLASS.getField("age"); });}@Testpublic void testGetDeclaredFields() {// getDeclaredFields 获取当前类中定义的所有字段（不能获取父类中定义的字段）Collection<String> declaredFieldNameList = Arrays.stream(USER_INFO_ENTITY_CLASS.getDeclaredFields()).map(Field::getName).collect(Collectors.toList());List<String> expectedDeclaredFieldNameList = Arrays.asList("name", "age");assertThat(declaredFieldNameList.size()).isEqualTo(expectedDeclaredFieldNameList.size());assertThat(declaredFieldNameList).containsAll(expectedDeclaredFieldNameList);}@Testpublic void testGetDeclaredField() throws NoSuchFieldException {assertThatExceptionOfType(NoSuchFieldException.class).isThrownBy(() -> { USER_INFO_ENTITY_CLASS.getDeclaredField("id"); });assertThat(USER_INFO_ENTITY_CLASS.getDeclaredField("name")).isNotNull();assertThat(USER_INFO_ENTITY_CLASS.getDeclaredField("age")).isNotNull();}
}

单元测试方法介绍：

testGetFields：使用getFields方法获取当前类及其父类中使用public定义的所有字段

testGetField：使用getField方法获取当前类及其父类中使用public定义的指定字段

testGetDeclaredFields：使用getDeclaredFields方法获取当前类中定义的所有字段

testGetDeclaredField：使用getDeclaredField方法获取当前类中定义的指定字段

三、Field类中方法的使用

在src/test/java目录的cn.horse.reflect包下新建FieldInstanceMethodTests测试类

FieldInstanceMethodTests类：

package cn.horse.reflect;import cn.horse.reflect.entity.*;
import org.junit.jupiter.api.Test;import java.lang.annotation.Annotation;
import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
import java.util.Arrays;
import java.util.Collection;
import java.util.List;
import java.util.stream.Collectors;import static org.assertj.core.api.Assertions.assertThat;
import static org.assertj.core.api.Assertions.assertThatExceptionOfType;public class FieldInstanceMethodTests {static final Class<UserInfoEntity> USER_INFO_ENTITY_CLASS = UserInfoEntity.class;@Testpublic void testGetDeclaringClass() throws NoSuchFieldException {assertThat(USER_INFO_ENTITY_CLASS.getField("id").getDeclaringClass()).isEqualTo(BaseEntity.class);assertThat(USER_INFO_ENTITY_CLASS.getDeclaredField("name").getDeclaringClass()).isEqualTo(UserInfoEntity.class);assertThat(USER_INFO_ENTITY_CLASS.getDeclaredField("age").getDeclaringClass()).isEqualTo(UserInfoEntity.class);}@Testpublic void testGetType() throws NoSuchFieldException {assertThat(USER_INFO_ENTITY_CLASS.getField("id").getType()).isEqualTo(Long.class);assertThat(USER_INFO_ENTITY_CLASS.getDeclaredField("name").getType()).isEqualTo(String.class);assertThat(USER_INFO_ENTITY_CLASS.getDeclaredField("age").getType()).isEqualTo(Integer.class);}@Testpublic void testGetName() throws NoSuchFieldException {assertThat(USER_INFO_ENTITY_CLASS.getField("id").getName()).isEqualTo("id");assertThat(USER_INFO_ENTITY_CLASS.getDeclaredField("name").getName()).isEqualTo("name");assertThat(USER_INFO_ENTITY_CLASS.getDeclaredField("age").getName()).isEqualTo("age");}@Testpublic void testGetModifiers() throws NoSuchFieldException {assertThat(Modifier.isPublic(USER_INFO_ENTITY_CLASS.getField("id").getModifiers())).isTrue();assertThat(Modifier.isPublic(USER_INFO_ENTITY_CLASS.getDeclaredField("name").getModifiers())).isTrue();assertThat(Modifier.isPrivate(USER_INFO_ENTITY_CLASS.getDeclaredField("age").getModifiers())).isTrue();}@Testpublic void testGetFieldValue() throws NoSuchFieldException, IllegalAccessException {UserInfoEntity userInfoEntity = new UserInfoEntity(200L, "张三", 10);assertThat(USER_INFO_ENTITY_CLASS.getField("id").get(userInfoEntity)).isEqualTo(200L);assertThat(USER_INFO_ENTITY_CLASS.getDeclaredField("name").get(userInfoEntity)).isEqualTo("张三");// 获取age字段Field ageField = USER_INFO_ENTITY_CLASS.getDeclaredField("age");assertThatExceptionOfType(IllegalAccessException.class).isThrownBy(() -> { ageField.get(userInfoEntity); });ageField.setAccessible(true);assertThat(ageField.get(userInfoEntity)).isEqualTo(10);}@Testpublic void testSetFieldValue() throws NoSuchFieldException, IllegalAccessException {UserInfoEntity userInfoEntity = new UserInfoEntity(200L, "张三", 10);USER_INFO_ENTITY_CLASS.getField("id").set(userInfoEntity, 300L);USER_INFO_ENTITY_CLASS.getDeclaredField("name").set(userInfoEntity, "李四");// 获取age字段Field ageField = USER_INFO_ENTITY_CLASS.getDeclaredField("age");assertThatExceptionOfType(IllegalAccessException.class).isThrownBy(() -> { ageField.set(userInfoEntity, 20); });ageField.setAccessible(true);ageField.set(userInfoEntity, 20);assertThat(userInfoEntity.getId()).isEqualTo(300L);assertThat(userInfoEntity.name).isEqualTo("李四");assertThat(userInfoEntity.getAge()).isEqualTo(20);}@Testpublic void testIdFieldGetAnnotations() throws NoSuchFieldException {Collection<Class<? extends Annotation>> annotationList = Arrays.stream(USER_INFO_ENTITY_CLASS.getField("id").getAnnotations()).map(Annotation::annotationType).collect(Collectors.toList());List<Class<? extends Annotation>> expectedAnnotationList = Arrays.asList(EntityAnnotation.class, InheritedEntityAnnotation.class);assertThat(annotationList.size()).isEqualTo(expectedAnnotationList.size());assertThat(annotationList).containsAll(expectedAnnotationList);}@Testpublic void testNameFieldGetAnnotations() throws NoSuchFieldException {Collection<Class<? extends Annotation>> annotationList = Arrays.stream(USER_INFO_ENTITY_CLASS.getField("name").getAnnotations()).map(Annotation::annotationType).collect(Collectors.toList());List<Class<? extends Annotation>> expectedAnnotationList = Arrays.asList(UserInfoEntityAnnotation.class);assertThat(annotationList.size()).isEqualTo(expectedAnnotationList.size());assertThat(annotationList).containsAll(expectedAnnotationList);}@Testpublic void testAgeFieldGetAnnotations() throws NoSuchFieldException {Collection<Class<? extends Annotation>> annotationList = Arrays.stream(USER_INFO_ENTITY_CLASS.getDeclaredField("age").getAnnotations()).map(Annotation::annotationType).collect(Collectors.toList());List<Class<? extends Annotation>> expectedAnnotationList = Arrays.asList(UserInfoEntityAnnotation.class);assertThat(annotationList.size()).isEqualTo(expectedAnnotationList.size());assertThat(annotationList).containsAll(expectedAnnotationList);}@Testpublic void testFieldGetAnnotationsByType() throws NoSuchFieldException {Collection<Class<? extends Annotation>> annotationList = Arrays.stream(USER_INFO_ENTITY_CLASS.getField("name").getAnnotationsByType(UserInfoEntityAnnotation.class)).map(Annotation::annotationType).collect(Collectors.toList());List<Class<? extends Annotation>> expectedAnnotationList = Arrays.asList(UserInfoEntityAnnotation.class);assertThat(annotationList.size()).isEqualTo(expectedAnnotationList.size());assertThat(annotationList).containsAll(expectedAnnotationList);}@Testpublic void testFieldGetAnnotation() throws NoSuchFieldException {assertThat(USER_INFO_ENTITY_CLASS.getField("id").getAnnotation(EntityAnnotation.class)).isNotNull();assertThat(USER_INFO_ENTITY_CLASS.getField("id").getAnnotation(InheritedEntityAnnotation.class)).isNotNull();assertThat(USER_INFO_ENTITY_CLASS.getField("id").getAnnotation(UserInfoEntityAnnotation.class)).isNull();assertThat(USER_INFO_ENTITY_CLASS.getField("name").getAnnotation(EntityAnnotation.class)).isNull();assertThat(USER_INFO_ENTITY_CLASS.getField("name").getAnnotation(InheritedEntityAnnotation.class)).isNull();assertThat(USER_INFO_ENTITY_CLASS.getField("name").getAnnotation(UserInfoEntityAnnotation.class)).isNotNull();assertThat(USER_INFO_ENTITY_CLASS.getDeclaredField("age").getAnnotation(EntityAnnotation.class)).isNull();assertThat(USER_INFO_ENTITY_CLASS.getDeclaredField("age").getAnnotation(InheritedEntityAnnotation.class)).isNull();assertThat(USER_INFO_ENTITY_CLASS.getDeclaredField("age").getAnnotation(UserInfoEntityAnnotation.class)).isNotNull();}@Testpublic void testIdFieldGetDeclaredAnnotations() throws NoSuchFieldException {Collection<Class<? extends Annotation>> annotationList = Arrays.stream(USER_INFO_ENTITY_CLASS.getField("id").getDeclaredAnnotations()).map(Annotation::annotationType).collect(Collectors.toList());List<Class<? extends Annotation>> expectedAnnotationList = Arrays.asList(EntityAnnotation.class, InheritedEntityAnnotation.class);assertThat(annotationList.size()).isEqualTo(expectedAnnotationList.size());assertThat(annotationList).containsAll(expectedAnnotationList);}@Testpublic void testNameFieldGetDeclaredAnnotations() throws NoSuchFieldException {Collection<Class<? extends Annotation>> annotationList = Arrays.stream(USER_INFO_ENTITY_CLASS.getDeclaredField("name").getDeclaredAnnotations()).map(Annotation::annotationType).collect(Collectors.toList());List<Class<? extends Annotation>> expectedAnnotationList = Arrays.asList(UserInfoEntityAnnotation.class);assertThat(annotationList.size()).isEqualTo(expectedAnnotationList.size());assertThat(annotationList).containsAll(expectedAnnotationList);}@Testpublic void testAgeFieldGetDeclaredAnnotations() throws NoSuchFieldException {Collection<Class<? extends Annotation>> annotationList = Arrays.stream(USER_INFO_ENTITY_CLASS.getDeclaredField("age").getDeclaredAnnotations()).map(Annotation::annotationType).collect(Collectors.toList());List<Class<? extends Annotation>> expectedAnnotationList = Arrays.asList(UserInfoEntityAnnotation.class);assertThat(annotationList.size()).isEqualTo(expectedAnnotationList.size());assertThat(annotationList).containsAll(expectedAnnotationList);}@Testpublic void testFieldGetDeclaredAnnotationsByType() throws NoSuchFieldException {Collection<Class<? extends Annotation>> annotationList = Arrays.stream(USER_INFO_ENTITY_CLASS.getDeclaredField("name").getDeclaredAnnotationsByType(UserInfoEntityAnnotation.class)).map(Annotation::annotationType).collect(Collectors.toList());List<Class<? extends Annotation>> expectedAnnotationList = Arrays.asList(UserInfoEntityAnnotation.class);assertThat(annotationList.size()).isEqualTo(expectedAnnotationList.size());assertThat(annotationList).containsAll(expectedAnnotationList);}@Testpublic void testFieldGetDeclaredAnnotation() throws NoSuchFieldException {assertThat(USER_INFO_ENTITY_CLASS.getField("id").getDeclaredAnnotation(EntityAnnotation.class)).isNotNull();assertThat(USER_INFO_ENTITY_CLASS.getField("id").getDeclaredAnnotation(InheritedEntityAnnotation.class)).isNotNull();assertThat(USER_INFO_ENTITY_CLASS.getField("id").getDeclaredAnnotation(UserInfoEntityAnnotation.class)).isNull();assertThat(USER_INFO_ENTITY_CLASS.getDeclaredField("name").getDeclaredAnnotation(EntityAnnotation.class)).isNull();assertThat(USER_INFO_ENTITY_CLASS.getDeclaredField("name").getDeclaredAnnotation(InheritedEntityAnnotation.class)).isNull();assertThat(USER_INFO_ENTITY_CLASS.getDeclaredField("name").getDeclaredAnnotation(UserInfoEntityAnnotation.class)).isNotNull();assertThat(USER_INFO_ENTITY_CLASS.getDeclaredField("age").getDeclaredAnnotation(EntityAnnotation.class)).isNull();assertThat(USER_INFO_ENTITY_CLASS.getDeclaredField("age").getDeclaredAnnotation(InheritedEntityAnnotation.class)).isNull();assertThat(USER_INFO_ENTITY_CLASS.getDeclaredField("age").getDeclaredAnnotation(UserInfoEntityAnnotation.class)).isNotNull();}
}

单元测试方法介绍：

testGetDeclaringClass：使用getDeclaringClass方法获取到字段所在类（声明类）的Class对象

testGetType：使用getType方法获取字段的类型

testGetName：使用getName方法获取字段的名称

testGetModifiers：使用getModifiers获取修饰符，并使用Modifier类提供的静态方法依次判断了是否使用了public、private、protected等修饰符

testGetFieldValue：使用get方法获取对象中字段的值，如果字段不可访问会抛出IllegalAccessException异常，使用setAccessible方法可以控制访问权限。

testSetFieldValue：使用set方法设置对象中字段的值，如果字段不可访问会抛出IllegalAccessException异常，使用setAccessible方法可以控制访问权限。

testIdFieldGetAnnotations：使用getAnnotations获取字段的所有注解

testNameFieldGetAnnotations：使用getAnnotations获取字段的所有注解

testAgeFieldGetAnnotations：使用getAnnotations获取字段的所有注解

testFieldGetAnnotationsByType：使用getAnnotationsByType从字段的注解中获取指定类型的注解

testFieldGetAnnotation：使用getAnnotation从字段的注解中获取指定类型的注解

testIdFieldGetDeclaredAnnotations：使用getDeclaredAnnotations获取字段的所有注解

testNameFieldGetDeclaredAnnotations：使用getDeclaredAnnotations获取字段的所有注解

testAgeFieldGetDeclaredAnnotations：使用getDeclaredAnnotations获取字段的所有注解

testFieldGetDeclaredAnnotationsByType：使用getDeclaredAnnotationsByType从字段的注解中获取指定类型的注解

testFieldGetDeclaredAnnotation：使用getDeclaredAnnotation从字段的注解中获取指定类型的注解