java - 使用注释创建 Callable

Question

我正在尝试制作一个类似于 https://github.com/ElgarL/TownyChat/blob/master/src/com/palmergames/bukkit/TownyChat/TownyChatFormatter.java 的系统

replacer.registerFormatReplacement(Pattern.quote("{worldname}"), new TownyChatReplacerCallable() {
        @Override
        public String call(String match, LocalTownyChatEvent event) throws Exception {
            return String.format(ChatSettings.getWorldTag(), event.getEvent().getPlayer().getWorld().getName());
        }
    });
    replacer.registerFormatReplacement(Pattern.quote("{town}"), new TownyChatReplacerCallable() {
        @Override
        public String call(String match, LocalTownyChatEvent event) throws Exception {
            return event.getResident().hasTown() ? event.getResident().getTown().getName() : "";
        }
    });

还有更多。

有没有办法使用注释来减少重复代码量，避免反射调用 call 方法，并且仅在注册期间使用它(如果有的话)？

我并不反对创建注释预处理器的想法，因为我已经计划这样做以实现自动生成文档。

Answer 1

假设您编写了一个小注释

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
@interface PatternHandler {
    String value();
}

并创建一个类似的类

class Callables {

    @PatternHandler("foo")
    public static final TownyChatReplacerCallable FOO = new TownyChatReplacerCallable() {
        @Override
        public String call(String match, String event) {
            return "This is foo handler called with " + match + "," + event;
        }
    };

    @PatternHandler("bar")
    public static final TownyChatReplacerCallable BAR = new TownyChatReplacerCallable() {
        @Override
        public String call(String match, String event) {
            return "This is foo handler called with " + match + "," + event;
        }
    };
}

现在，您可以获取整个类，甚至包含这些静态字段的多个类，并将其传递给某个注册表方法，该方法以反射方式迭代该类中的每个字段，如果它是带注释的可调用函数，则注册它。

class AnnotationRegistry {
    public static void register(String pattern, TownyChatReplacerCallable handler) {}

    public static void register(Class<?> clazz) {
        // only fields declared by this class, not inherited ones (static fields can't be inherited)
        Field[] fields = clazz.getDeclaredFields();
        for (Field field : fields) {
            // must have that annotation
            PatternHandler annotation = field.getAnnotation(PatternHandler.class);
            if (annotation != null) {
                // must be static
                if (!Modifier.isStatic(field.getModifiers())) {
                    System.out.println("Field must be static:" + field.getName());
                    continue;
                }
                // get content of that field
                try {
                    Object object = field.get(null);
                    // must be != null and a callable
                    if (object instanceof TownyChatReplacerCallable) {
                        register(annotation.value(), (TownyChatReplacerCallable) object);
                    } else {
                        System.out.println("Field must be instanceof TownyChatReplacerCallable:"  + field.getName());
                    }
                } catch (IllegalArgumentException e) {
                    e.printStackTrace();
                } catch (IllegalAccessException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

这将为您节省一些代码，并且在运行时不会有速度劣势，因为不需要使用反射来调用这些可调用对象。

完整示例在这里:http://ideone.com/m3PPcY

除了使用静态字段之外，如果将类的实例传递到注册表，您还可以使用非静态字段，然后像 Object object = field.get(instance); 一样使用它，而不是 null。

此外，与字段不同，相同的方法可以使用需要编写的代码更少的方法:

@PatternHandler("foo")
public static String fooMethod(String match, String event) {
    return "This is foo handler called with " + match + "," + event;
}

注册中心将查找所有 Method s。然后例如将它们包裹在

class MethodAdapter implements TownyChatReplacerCallable {
    private final Method method;
    public MethodAdapter(Method m) {
        method = m;
    }
    @Override
    public String call(String match, String event) {
        try {
            return (String) method.invoke(null, match, event);
        } catch (Exception e) {
            e.printStackTrace();
            return "OMGZ";
        }
    }
}

并照常继续。但要注意:反射调用方法可能比通过代码直接调用慢 - 仅百分之几，无需担心

方法的完整示例:http://ideone.com/lMJsrl