Lombok之@EqualsAndHashCode使用

一. 为什么要用@EqualsAndHashCode?

在java.lang.Object中有两个实例方法——equals和hashCode。这两个方法就像孪生兄弟一样，重写equals，就要重写hashCode。至于为什么？可以看这篇博客《java中==,equals,hashcode》补补课。现在，我们来看看如何手动重写equals和hashCode方法。可以，使用Intellij IDEA的快捷键生成equals和hashCode方法：

在Template的下拉框中有着不同的选项用于生成equals和hashCode方法，我们选择java.util.Objects.equals and hashCode (java 7+)，利用jdk1.7 的Objects工具类。上图的可选项1是指是否将从父类继承的字段包含到生成的equals方法中，可选项2是指是否使用getter方法，如果不是getter方法（student.getName()），则使用对象.成员变量（student.name）的形式直接访问成员变量。

@Getter
@Setter
@ToString
// @EqualsAndHashCode
public class Student {

    private String name;

    private Integer age;

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Student student = (Student) o;
        return Objects.equals(name, student.name) &&
                Objects.equals(age, student.age);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name, age);
    }
}

可以看到生成的代码中使用Objects的类方法equals(Object o)和hash()。同样的问题，生成的方法占据了不小的篇幅。

二. @EqualsAndHashCode如何使用？

@EqualsAndHashCode的使用很简单，只需在Student类加上@EqualsAndHashCode后，编译，再查看编译后的文件。

public boolean equals(Object o) {
        if (o == this) {
            return true;
        } else if (!(o instanceof Student)) {
            return false;
        } else {
            Student other = (Student)o;
            if (!other.canEqual(this)) {
                return false;
            } else {
                Object this$name = this.getName();
                Object other$name = other.getName();
                if (this$name == null) {
                    if (other$name != null) {
                        return false;
                    }
                } else if (!this$name.equals(other$name)) {
                    return false;
                }

                Object this$age = this.getAge();
                Object other$age = other.getAge();
                if (this$age == null) {
                    if (other$age != null) {
                        return false;
                    }
                } else if (!this$age.equals(other$age)) {
                    return false;
                }

                return true;
            }
        }
    }

    protected boolean canEqual(Object other) {
        return other instanceof Student;
    }

    public int hashCode() {
        int PRIME = true;
        int result = 1;
        Object $name = this.getName();
        int result = result * 59 + ($name == null ? 43 : $name.hashCode());
        Object $age = this.getAge();
        result = result * 59 + ($age == null ? 43 : $age.hashCode());
        return result;
    }

出于文章篇幅的考虑，只贴出了由@EqualsAndHashCode注解生成的代码。可以看到生成访问控制符为protected的canEqual(Object other)方法。

可以看到，只要属性全部相等，student和student1的equals方法就返回true。但是 == 永远判断是内存地址，所以当某个类重写了equals方法后，就要使用equals方法去做判断。

三. @EqualsAndHashCode源码

package lombok;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * Generates implementations for the {@code equals} and {@code hashCode} methods inherited by all objects, based on relevant fields.
 * 

 * Complete documentation is found at the project lombok features page for @EqualsAndHashCode.
 */
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.SOURCE)
public @interface EqualsAndHashCode {
	/**
	 * Any fields listed here will not be taken into account in the generated {@code equals} and {@code hashCode} implementations.
	 * Mutually exclusive with {@link #of()}.
	 * 

	 * Will soon be marked {@code @Deprecated}; use the {@code @EqualsAndHashCode.Exclude} annotation instead.
	 * 
	 * @return A list of fields to exclude.
	 */
	String[] exclude() default {};
	
	/**
	 * If present, explicitly lists the fields that are to be used for identity.
	 * Normally, all non-static, non-transient fields are used for identity.
	 * 

	 * Mutually exclusive with {@link #exclude()}.
	 * 

	 * Will soon be marked {@code @Deprecated}; use the {@code @EqualsAndHashCode.Include} annotation together with {@code @EqualsAndHashCode(onlyExplicitlyIncluded = true)}.
	 * 
	 * @return A list of fields to use (default: all of them).
	 */
	String[] of() default {};
	
	/**
	 * Call on the superclass's implementations of {@code equals} and {@code hashCode} before calculating for the fields in this class.
	 * default: false
	 * 
	 * @return Whether to call the superclass's {@code equals} implementation as part of the generated equals algorithm.
	 */
	boolean callSuper() default false;
	
	/**
	 * Normally, if getters are available, those are called. To suppress this and let the generated code use the fields directly, set this to {@code true}.
	 * default: false
	 * 
	 * @return If {@code true}, always use direct field access instead of calling the getter method.
	 */
	boolean doNotUseGetters() default false;
	
	/**
	 * Any annotations listed here are put on the generated parameter of {@code equals} and {@code canEqual}.
	 * This is useful to add for example a {@code Nullable} annotation.

	 * The syntax for this feature depends on JDK version (nothing we can do about that; it's to work around javac bugs).

	 * up to JDK7:

	 *  {@code @EqualsAndHashCode(onParam=@__({@AnnotationsGoHere}))}

	 * from JDK8:

	 *  {@code @EqualsAndHashCode(onParam_={@AnnotationsGohere})} // note the underscore after {@code onParam}.
	 *  
	 * @return List of annotations to apply to the generated parameter in the {@code equals()} method.
	 */
	AnyAnnotation[] onParam() default {};
	
	/**
	 * Placeholder annotation to enable the placement of annotations on the generated code.
	 * @deprecated Don't use this annotation, ever - Read the documentation.
	 */
	@Deprecated
	@Retention(RetentionPolicy.SOURCE)
	@Target({})
	@interface AnyAnnotation {}
	
	/**
	 * Only include fields and methods explicitly marked with {@code @EqualsAndHashCode.Include}.
	 * Normally, all (non-static, non-transient) fields are included by default.
	 * 
	 * @return If {@code true}, don't include non-static non-transient fields automatically (default: {@code false}).
	 */
	boolean onlyExplicitlyIncluded() default false;
	
	/**
	 * If present, do not include this field in the generated {@code equals} and {@code hashCode} methods.
	 */
	@Target(ElementType.FIELD)
	@Retention(RetentionPolicy.SOURCE)
	public @interface Exclude {}
	
	/**
	 * Configure the behaviour of how this member is treated in the {@code equals} and {@code hashCode} implementation; if on a method, include the method's return value as part of calculating hashCode/equality.
	 */
	@Target({ElementType.FIELD, ElementType.METHOD})
	@Retention(RetentionPolicy.SOURCE)
	public @interface Include {
		/**
		 * Defaults to the method name of the annotated member.
		 * If on a method and the name equals the name of a default-included field, this member takes its place.
		 * 
		 * @return If present, this method serves as replacement for the named field.
		 * 是指当一个方法的名称和变量名相同时，包含方法进入equals，不包含字段。
		 */
		String replaces() default "";
	}
}

• 元注解：@Target(ElementType.TYPE)只能在类上使用，@Retention(RetentionPolicy.SOURCE)只在代码里保留。
• 注解属性：

1. exclude，of不用了解，准备弃用了，使用EqualsAndHashCode.Exclude和EqualsAndHashCode.Include代替；
2. callSuper表示是否将父类的equals和hashCode方法加到该子类的equals和hashCode方法中；
3. doNotUseGetters表示是否使用getter访问成员变量；
4. onParam参考Lombok实验室之onX使用；
5. onlyExplicitlyIncluded为false时，所有的非静态和非瞬态的字段都会被包含进equals和hashCode方法中；为true时，只有在字段上明确使用了EqualsAndHashCode.Include注解才会被包含进equals和hashCode方法中。

四. 特别说明

本文已经收录在Lombok注解系列文章总览中，并继承上文中所提的特别说明。
源码地址：gitee