本文参考
在《Effective Java》第三版第十条"Obey the general contract when overriding equals"中提到google的AutoValue框架能够自动生成equals()方法，实际上这个框架的作用不仅仅限于生成equals()方法那么简单，它还能够使值类通过静态工厂方法构建实例，并实现Builder构建者模式，省去了程序员对值类的重复性工作
github地址：https://github.com/google/auto/blob/master/value/userguide/index.md
环境
idea 2020.1 + AutoValue 1.7.1
Maven配置
我们需要同时配置auto-value-annotations和auto-value两个dependency
<dependency>
<groupId>com.google.auto.value</groupId>
<artifactId>auto-value-annotations</artifactId>
<version>1.7.1</version>
<scope>provided</scope>
</dependency>
<dependency>
<groupId>com.google.auto.value</groupId>
<artifactId>auto-value</artifactId>
<version>1.7.1</version>
<optional>true</optional>
<scope>provided</scope>
</dependency>
Idea配置
在Build,Execution,Deployment -> Complier -> Annotation Processor中勾选Enable annotation processing
默认的Production source directory和Test sources directory不需要更改
基本用法
如下代码所示，构造Person抽象类，create()静态工厂方法，和抽象的字段方法
@AutoValue
abstract class Person {
abstract String getName();
abstract int getAge();
static Person create(String name, int age) {
return new AutoValue_Person(name, age);
}
}
此时显然还没有AutoValue_Person这个类（固定的前缀写法），因此idea会报错，但是我们暂时不用担心这个问题
编写一个简单的测试类，测试方法如下
@Test
public void test() {
Person person = Person.create("kuluo", 18);
assertEquals("kuluo", person.name());
}
可以先对源代码进行编译而不运行，编译结束后可以看到AutoValue_Person的报错消失
我们可以在默认的targetgenerated-sourcesannotations或targetgenerated-test-sources est-annotations文件夹中看到生成的值类
@Generated("com.google.auto.value.processor.AutoValueProcessor")
final class AutoValue_Person extends Person {
private final String name;
private final int age;
AutoValue_Person(String name, int age) {
if (name == null) {
throw new NullPointerException("Null name");
}
this.name = name;
this.age = age;
}
@Override
String getName() {
return name;
}
@Override
int getAge() {
return age;
}
@Override
public String toString() {
return "Person{"
+ "name=" + name + ", "
+ "age=" + age
+ "}";
}
@Override
public boolean equals(Object o) {
if (o == this) {
return true;
}
if (o instanceof Person) {
Person that = (Person) o;
return this.name.equals(that.name()) && this.age == that.age();
}
return false;
}
@Override
public int hashCode() {
int h$ = 1;
h$ *= 1000003;
h$ ^= name.hashCode();
h$ *= 1000003;
h$ ^= age;
return h$;
}
}
注意：
- 值类被声明为final类型，无法再被继承
- 值类不具备setter方法，实例被创建后就无法被更改
- 若在构造实例时允许传入可变类型的值，如List<String>和String[]，则需要在Guava中选择对应的不可变类型，并更改create()静态工厂方法
check if the mutable type has a corresponding immutable cousin. For example, the types List<String> and String[] have the immutable counterpart ImmutableList<String> in Guava. If so, use the immutable type for your property, and only accept the mutable type during construction
@AutoValue
abstract class Person {
abstract ImmutableList<String> getName();
abstract int getAge();
static Person create(List<String> name, int age) {
return new AutoValue_Person(ImmutableList.copyOf(name), age);
}
}
- 值类在构建实例时会检查每一个字段是否为null，若某字段为null，则抛出空指针异常
- 若允许某个字段为null，则必须在抽象类create()静态工厂方法的声明中，为该字段和它对应的getter方法同时添加@Nullable注解
if @Nullable is only added to the parameter in create (or similarly the setter method of AutoValue.Builder), but not the corresponding accessor method, it won't have any effect.
@AutoValue
abstract class Person {
@Nullable abstract String getName();
abstract int getAge();
static Person create(@Nullable String name, int age) {
return new AutoValue_Person(name, age);
}
}
下面仅展示发生变化的方法，我们可以看到在equals()方法和hashCode()方法中也自动增加了对null的判断
AutoValue_Person(@Nullable String name, int age) {
this.name = name;
this.age = age;
}
@Nullable
@Override
String getName() {
return name;
}
@Override
public boolean equals(Object o) {
if (o == this) {
return true;
}
if (o instanceof Person) {
Person that = (Person) o;
return (this.name == null ? that.getName() == null : this.name.equals(that.getName()))
&& this.age == that.getAge();
}
return false;
}
@Override
public int hashCode() {
int h$ = 1;
h$ *= 1000003;
h$ ^= (name == null) ? 0 : name.hashCode();
h$ *= 1000003;
h$ ^= age;
return h$;
}
Builder构建者模式用法
涉及抽象静态内部类Builder，并为它添加@AutoValue.Builder注解
@AutoValue
abstract class PersonWithBuilder {
abstract String getName();
abstract int getAge();
static Builder builder() {
return new AutoValue_PersonWithBuilder.Builder();
}
@AutoValue.Builder
abstract static class Builder {
abstract Builder name(String name);
abstract Builder age(int age);
abstract PersonWithBuilder build();
}
}
编写一个简单的测试类，测试代码如下：
@Test
public void testWithBuilder() {
PersonWithBuilder personWithBuilder = PersonWithBuilder
.builder()
.name("kuluo")
.age(18)
.build();
assertEquals("kuluo", personWithBuilder.getName());
assertEquals(18, personWithBuilder.getAge());
}
编译运行后生成AutoValue_PersonWithBuilder类
@Generated("com.google.auto.value.processor.AutoValueProcessor")
final class AutoValue_PersonWithBuilder extends PersonWithBuilder {
private final String name;
private final int age;
private AutoValue_PersonWithBuilder(String name, int age) {
this.name = name;
this.age = age;
}
@Override
String getName() { return name; }
@Override
int getAge() { return age; }
@Override
public String toString() {
return "PersonWithBuilder{"
+ "name=" + name + ", "
+ "age=" + age
+ "}";
}
@Override
public boolean equals(Object o) {
if (o == this) { return true; }
if (o instanceof PersonWithBuilder) {
PersonWithBuilder that = (PersonWithBuilder) o;
return this.name.equals(that.getName()) && this.age == that.getAge();
}
return false;
}
@Override
public int hashCode() {
int h$ = 1;
h$ *= 1000003;
h$ ^= name.hashCode();
h$ *= 1000003;
h$ ^= age;
return h$;
}
static final class Builder extends PersonWithBuilder.Builder {
private String name;
private Integer age;
Builder() {
}
@Override
PersonWithBuilder.Builder name(String name) {
if (name == null) {
throw new NullPointerException("Null name");
}
this.name = name;
return this;
}
@Override
PersonWithBuilder.Builder age(int age) {
this.age = age;
return this;
}
@Override
PersonWithBuilder build() {
String missing = "";
if (this.name == null) {
missing += " name";
}
if (this.age == null) {
missing