本文參考
在《Effective Java》第三版第十條"Obey the general contract when overriding equals"中提到google的AutoValue框架能夠自動生成equals()方法，實際上這個框架的作用不僅僅限於生成equals()方法那么簡單，它還能夠使值類通過靜態工廠方法構建實例，並實現Builder構建者模式，省去了程序員對值類的重復性工作
github地址：https://github.com/google/auto/blob/master/value/userguide/index.md
環境
idea 2020.1 + AutoValue 1.7.1
Maven配置
我們需要同時配置auto-value-annotations和auto-value兩個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中勾選Enable annotation processing
默認的Production source directory和Test sources directory不需要更改
基本用法
如下代碼所示，構造Person抽象類，create()靜態工廠方法，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);
}
}
此時顯然還沒有AutoValue_Person這個類（固定的前綴寫法），因此idea會報錯，但是我們暫時不用擔心這個問題
編寫一個簡單的測試類，測試方法如下
@Test
public void test() {
Person person = Person.create("kuluo", 18);
assertEquals("kuluo", person.name());
}
可以先對源代碼進行編譯而不運行，編譯結束后可以看到AutoValue_Person的報錯消失
我們可以在默認的target\generated-sources\annotations或target\generated-test-sources\test-annotations文件夾中看到生成的值類
@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$;
}
}
注意：
- 值類被聲明為final類型，無法再被繼承
- 值類不具備setter方法，實例被創建后就無法被更改
- 若在構造實例時允許傳入可變類型的值，如List<String>和String[]，則需要在Guava中選擇對應的不可變類型，並更改create()靜態工廠方法
check if the mutable type has a corresponding immutable cousin. For example, the types List<String> and String[] have the immutable counterpart ImmutableList<String> in Guava. If so, use the immutable type for your property, and only accept the mutable type during construction
@AutoValue
abstract class Person {
abstract ImmutableList<String> getName();
abstract int getAge();
static Person create(List<String> name, int age) {
return new AutoValue_Person(ImmutableList.copyOf(name), age);
}
}
- 值類在構建實例時會檢查每一個字段是否為null，若某字段為null，則拋出空指針異常
- 若允許某個字段為null，則必須在抽象類create()靜態工廠方法的聲明中，為該字段和它對應的getter方法同時添加@Nullable注解
if @Nullable is only added to the parameter in create (or similarly the setter method of AutoValue.Builder), but not the corresponding accessor method, it won't have any effect.
@AutoValue
abstract class Person {
@Nullable abstract String getName();
abstract int getAge();
static Person create(@Nullable String name, int age) {
return new AutoValue_Person(name, age);
}
}
下面僅展示發生變化的方法，我們可以看到在equals()方法和hashCode()方法中也自動增加了對null的判斷
AutoValue_Person(@Nullable String name, int age) {
this.name = name;
this.age = age;
}
@Nullable
@Override
String getName() {
return name;
}
@Override
public boolean equals(Object o) {
if (o == this) {
return true;
}
if (o instanceof Person) {
Person that = (Person) o;
return (this.name == null ? that.getName() == null : this.name.equals(that.getName()))
&& this.age == that.getAge();
}
return false;
}
@Override
public int hashCode() {
int h$ = 1;
h$ *= 1000003;
h$ ^= (name == null) ? 0 : name.hashCode();
h$ *= 1000003;
h$ ^= age;
return h$;
}
Builder構建者模式用法
涉及抽象靜態內部類Builder，並為它添加@AutoValue.Builder注解
@AutoValue
abstract class PersonWithBuilder {
abstract String getName();
abstract int getAge();
static Builder builder() {
return new AutoValue_PersonWithBuilder.Builder();
}
@AutoValue.Builder
abstract static class Builder {
abstract Builder name(String name);
abstract Builder age(int age);
abstract PersonWithBuilder build();
}
}
編寫一個簡單的測試類，測試代碼如下：
@Test
public void testWithBuilder() {
PersonWithBuilder personWithBuilder = PersonWithBuilder
.builder()
.name("kuluo")
.age(18)
.build();
assertEquals("kuluo", personWithBuilder.getName());
assertEquals(18, personWithBuilder.getAge());
}
編譯運行后生成AutoValue_PersonWithBuilder類
@Generated("com.google.auto.value.processor.AutoValueProcessor")
final class AutoValue_PersonWithBuilder extends PersonWithBuilder {
private final String name;
private final int age;
private AutoValue_PersonWithBuilder(String name, int age) {
this.name = name;
this.age = age;
}
@Override
String getName() { return name; }
@Override
int getAge() { return age; }
@Override
public String toString() {
return "PersonWithBuilder{"
+ "name=" + name + ", "
+ "age=" + age
+ "}";
}
@Override
public boolean equals(Object o) {
if (o == this) { return true; }
if (o instanceof PersonWithBuilder) {
PersonWithBuilder that = (PersonWithBuilder) o;
return this.name.equals(that.getName()) && this.age == that.getAge();
}
return false;
}
@Override
public int hashCode() {
int h$ = 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 += " age";