Lombok之@ToString使用

在java.lang.Object中有个实例方法toString，这个方法的作用是一个对象的自我描述。在源码中有这样一句注释，It is recommended that all subclasses override this method.即推荐所有的子类重新该方法。因为该方法在Object中的实现是返回字符串——类名和该对象的hashCode用“@”符连接起来的字符串，不具有可读性。所以，需要重写该方法，使得该方法能够清楚地表述自己的每一个成员变量。现在，我们在已经创建的Student类，重写该方法。

可以看到重写后的方法占据了不小的篇幅，并且当我们新增一个成员变量后，需要再去维护这个方法的实现。那有没有这样一个注解，可以自动生成toString方法？答案就是@ToString。

@ToString的使用很简单，我们只需将它加到Student类上即可。

测试使用 将项目使用maven编译后，再打开Student.class文件。 可以看到，反编译后的文件已经重写了toString方法。

* Complete documentation is found at the project lombok features page for @ToString. */ @Target(ElementType.TYPE) @Retention(RetentionPolicy.SOURCE) public @interface ToString { /** * Include the name of each field when printing it. * default: true * * @return Whether or not to include the names of fields in the string produced by the generated {@code toString()}. * 是否打印出字段名 */ boolean includeFieldNames() default true; /** * Any fields listed here will not be printed in the generated {@code toString} implementation. * Mutually exclusive with {@link #of()}. *

* Will soon be marked {@code @Deprecated}; use the {@code @ToString.Exclude} annotation instead. * 不久后就被弃用，用@ToString.Exclude注解代替 * @return A list of fields to exclude. */ String[] exclude() default {}; /** * If present, explicitly lists the fields that are to be printed. * Normally, all non-static fields are printed. *

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

* Will soon be marked {@code @Deprecated}; use the {@code @ToString.Include} annotation together with {@code @ToString(onlyExplicitlyIncluded = true)}. * 不久后将被弃用，用@ToString.Include注解代替并且设置@ToString(onlyExplicitlyIncluded = true) * * @return A list of fields to use (default: all of them). */ String[] of() default {}; /** * Include the result of the superclass's implementation of {@code toString} in the output. * default: false * * @return Whether to call the superclass's {@code toString} implementation as part of the generated toString 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; /** * Only include fields and methods explicitly marked with {@code @ToString.Include}. * Normally, all (non-static) fields are included by default. * * @return If {@code true}, don't include non-static fields automatically (default: {@code false}). * 如果为真，则不会自动将非静态的字段包含到toString方法中 */ boolean onlyExplicitlyIncluded() default false; /** * If present, do not include this field in the generated {@code toString}. */ @Target(ElementType.FIELD) @Retention(RetentionPolicy.SOURCE) public @interface Exclude {} /** * Configure the behaviour of how this member is rendered in the {@code toString}; if on a method, include the method's return value in the output. */ @Target({ElementType.FIELD, ElementType.METHOD}) @Retention(RetentionPolicy.SOURCE) public @interface Include { // /** If true and the return value is {@code null}, omit this member entirely from the {@code toString} output. */ // boolean skipNull() default false; // -- We'll add it later, it requires a complete rework on the toString code we generate. /** * Higher ranks are printed first. Members of the same rank are printed in the order they appear in the source file. * * @return ordering within the generating {@code toString()}; higher numbers are printed first. */ int rank() default 0; /** * Defaults to the field / method name of the annotated member. * If the name equals the name of a default-included field, this member takes its place. * * @return The name to show in the generated {@code toString()}. Also, if this annotation is on a method and the name matches an existing field, it replaces that field. */ String name() default ""; } }

includeFieldNames参数默认值为true，可以为toString()方法的输出增加一些清晰度（即打印出字段名称）。

默认情况下，将打印所有非静态字段。如果要跳过某些字段，可以用注释这些字段@ToString.Exclude。另外，可以使用精确指定要被toString方法包含的字段，先设置@ToString(onlyExplicitlyIncluded = true)，然后使用@ToString.Include标记每个要包含的字段。

通过设置callSuper为true，可以将超类实现toString的输出包含到输出中。请注意，toString()的默认实现java.lang.Object几乎没有意义，因此除非你主动继承了另一个类，否则你这样做没有意义。

您可以更改用于标识成员的名称@ToString.Include(name = “some other name”)，name相当于给字段起别名；也可以通过更改成员的打印顺序@ToString.Include(rank = -1)。没有等级的成员被认为是等级0，等级数字大的成员首先被打印，等级相同的成员以它们在源文件中出现的顺序打印。

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