我有一个函数可以提供用给定注释注释的类映射
void <A extends Annotation> ImmutableMap<Class<?>, A> find(Class<A> annotation, String packageBase) {
final ClassLoader loader = Thread.currentThread().getContextClassLoader();
return ClassPath.from(loader).getTopLevelClassesRecursive(packageBase).stream()
.filter(x -> x.load().getAnnotation(annotation) != null)
.collect(Collectors.collectingAndThen(Collectors
.toMap(ClassPath.ClassInfo::load, x -> x.load().getAnnotation(annotation)), ImmutableMap::copyOf));
}
我想做一个带有缓存的通用提供者,就像下面的例子
@Singleton
public class AnnotatedClassProvider {
private final Map<Class<? extends Annotation>, ImmutableMap<Class<?>, Object>> cache;
private final String basePackage;
public AnnotatedClassProvider(String basePackage) {
this.basePackage = basePackage;
this.cache = Maps.newHashMap();
}
public <A extends Annotation> ImmutableMap<Class<?>, A> get(Class<A> annotation) {
ImmutableMap<Class<?>, A> cached = cache.get(annotation);
if (cached != null)
return cached;
final ClassLoader loader = Thread.currentThread().getContextClassLoader();
cached = ClassPath.from(loader).getTopLevelClassesRecursive(basePackage).stream()
.filter(x -> x.load().getAnnotation(annotation) != null)
.collect(Collectors.collectingAndThen(Collectors
.toMap(ClassPath.ClassInfo::load, x -> x.load().getAnnotation(annotation)), ImmutableMap::copyOf));
this.cache.put(annotation, cached);
return (cached);
}
}
我的问题:我找不到替换 Object
的方法通过通用 A
就像在 get
函数,对于以下 map :
private final Map<Class<? extends Annotation>, ImmutableMap<Class<?>, Object>> cache;
编辑:
当我没有指定 map 泛型但我必须在 get 方法中强制转换时,它会编译。有没有办法避免这种情况?
private final Map<Class<? extends Annotation>, ImmutableMap> cache;
我觉得应该是这样的
private final <A extends Annotation> Map<Class<A>, ImmutableMap<Class<?>, A>> cache;
最佳答案
没有强制转换似乎是不可能的,所以下面几行解决了我的问题:
private final Map<Class<? extends Annotation>, ImmutableMap> cache;
public <A extends Annotation> ImmutableMap<Class<?>, A> get(Class<A> annotation) {
@SuppressWarnings("unchecked")
ImmutableMap<Class<?>, A> cached = cache.get(annotation);
...
}
对于有趣的人,这就是我的提供者现在的样子
界面
public interface AnnotatedClassProvider {
<A extends Annotation> ImmutableMap<Class<?>, A> get(Class<A> annotation) throws IOException;
}
抽象类
public abstract class AbstractAnnotatedClassProvider implements AnnotatedClassProvider {
private final Map<Class<? extends Annotation>, ImmutableMap> cache;
public AbstractAnnotatedClassProvider() {
this.cache = Maps.newHashMap();
}
protected final <A extends Annotation> ImmutableMap<Class<?>, A> find(Class<A> annotation, @Nullable String basePackage) throws IOException {
@SuppressWarnings("unchecked")
ImmutableMap<Class<?>, A> cached = cache.get(annotation);
if (cached != null)
return cached;
ClassPath classPath = ClassPath.from(Thread.currentThread().getContextClassLoader());
ImmutableSet<ClassPath.ClassInfo> set = basePackage == null
? classPath.getAllClasses()
: classPath.getTopLevelClasses(basePackage);
cached = set.stream()
.filter(x -> x.load().getAnnotation(annotation) != null)
.collect(Collectors.collectingAndThen(Collectors
.toMap(ClassPath.ClassInfo::load, x -> x.load().getAnnotation(annotation)), ImmutableMap::copyOf));
this.cache.put(annotation, cached);
return (cached);
}
}
实现
public final class Providers {
public static AnnotatedClassProvider newBased(String basePackage) {
return new AbstractAnnotatedClassProvider() {
@Override
public <A extends Annotation> ImmutableMap<Class<?>, A> get(Class<A> annotation) throws IOException {
return super.find(annotation, basePackage);
}
};
}
public static AnnotatedClassProvider newSimple() {
return new AbstractAnnotatedClassProvider() {
@Override
public <A extends Annotation> ImmutableMap<Class<?>, A> get(Class<A> annotation) throws IOException {
return super.find(annotation, null);
}
};
}
}
示例
@Retention(RUNTIME)
@Target(ElementType.TYPE)
public @interface Controller {
String value();
}
package com.test
@Controller("mainController")
public class Main {
public static void main(String[] args) {
AnnotatedClassProvider provider = Providers.newBased("com.test");
Map<Class<?>, Controller> classes = provider.get(Controller.class);
classes.forEach((x, y ) -> System.out.println(String.format("Class: %s annotated with %s with value %s",
x.getName(), y.getClass().getName(), y.value())));
}
}
输出:类:Main.java,用值为 mainController 的 Controller.class 注释
感谢所有评论。
关于java - 如何将java泛型从一个方法应用到同一个类中的一个字段,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/45232052/