Question

我有一个采用功能参数的方法，例如一个Runnable。由于它是一个库方法，我希望它使用从函数参数派生的记录器。对函数参数调用getClass对正常类很好，对于嵌套或匿名类我可以getEnclosingClass;但如果它是一个lambda表达式，它会返回一个含有$$Lambda$的模糊名称，我可以像这样手动剥离：

Class<?> type = runnable.getClass();
String canonical = type.getCanonicalName();
int lambdaOffset = canonical.indexOf("$$Lambda$");
if (lambdaOffset > 0) {
    try {
        type = Class.forName(canonical.substring(0, lambdaOffset));
    } catch (ClassNotFoundException e) {
        // strange, but we can stick to the type we already have
    }
}

正如您所看到的，这不是很优雅，可能不便携。我已尝试getEnclosingClass，getEnclosingMethod和getEnclosingConstructor，但他们都返回null。

有什么想法吗？

Answer 1

正如Tassos Bassoukos已经提到的那样，它是设计的。

Lambda（类）的字节码是在运行时生成的。所以你得到的是班级的实际名称。名称生成为target class name + "$$Lambda$" + a counter。

找一个小片段进行演示。

package sub.optimal;
import static java.lang.System.out;

public class EnclosingClass {

    static class InnerRunnable implements Runnable {

        @Override
        public void run() {
            out.println("--- inner class");
        }
    }

    public static void main(String... args) {
        showIdentity(() -> System.out.println("--- lambda 1"));
        showIdentity(() -> System.out.println("--- lambda 2"));
        showIdentity(new InnerRunnable());
        showIdentity(new Runnable() {
            @Override
            public void run() {
                out.println("--- anonymous class");
            }
        });
    }

    private static void showIdentity(Runnable runnable) {
        runnable.run();
        Class<? extends Runnable> clazz = runnable.getClass();
        out.printf("class name     : %s%n", clazz.getName());
        out.printf("class hashcode : %s%n", clazz.hashCode());
        out.printf("canonical name : %s%n", clazz.getCanonicalName());
        out.printf("enclosing class: %s%n", clazz.getEnclosingClass());
        out.println();
    }
}

<强>输出

--- lambda 1
class name     : sub.optimal.EnclosingClass$$Lambda$1/2147972
class hashcode : 2147972
canonical name : sub.optimal.EnclosingClass$$Lambda$1/2147972
enclosing class: null

--- lambda 2
class name     : sub.optimal.EnclosingClass$$Lambda$2/10376386
class hashcode : 10376386
canonical name : sub.optimal.EnclosingClass$$Lambda$2/10376386
enclosing class: null

--- inner class
class name     : sub.optimal.EnclosingClass$InnerRunnable
class hashcode : 28014437
canonical name : sub.optimal.EnclosingClass.InnerRunnable
enclosing class: class sub.optimal.EnclosingClass

--- anonymous class
class name     : sub.optimal.EnclosingClass$1
class hashcode : 19451386
canonical name : null
enclosing class: class sub.optimal.EnclosingClass

Answer 2

我在benjiweber找到了一个很酷的解决方案。归结为将lamda序列化为AddIdentityServer().AddConfigurationStore(...)然后得到它的声明类：

java.lang.invoke.SerializedLambda

这是很多代码，但我的机器上的开销大约是0.003毫秒，这对大多数用例来说都没问题。

你可以做其他很酷的事情：

private static final int COUNT = 1_000_000;
private static boolean first = true;

public static void main(String[] args) {
    long t = System.currentTimeMillis();
    for (int i = 0; i < COUNT; i++) {
        showIdentity(() -> {
        });
    }
    String time = NumberFormat.getNumberInstance().format((double) (System.currentTimeMillis() - t) / COUNT);
    System.out.println("time per call: " + time + "ms");
}

public interface MethodAwareRunnable extends Runnable, Serializable {}

private static void showIdentity(MethodAwareRunnable consumer) {
    consumer.run();
    String name = name(consumer);
    if (first) {
        first = false;
        Class<?> clazz = consumer.getClass();
        System.out.printf("class name     : %s%n", clazz.getName());
        System.out.printf("class hashcode : %s%n", clazz.hashCode());
        System.out.printf("canonical name : %s%n", clazz.getCanonicalName());
        System.out.printf("enclosing class: %s%n", clazz.getEnclosingClass());
        System.out.printf("lambda name    : %s%n", name);
    }
}

private static String name(Object consumer) {
    return method(consumer).getDeclaringClass().getName();
}

private static SerializedLambda serialized(Object lambda) {
    try {
        Method writeMethod = lambda.getClass().getDeclaredMethod("writeReplace");
        writeMethod.setAccessible(true);
        return (SerializedLambda) writeMethod.invoke(lambda);
    } catch (Exception e) {
        throw new RuntimeException(e);
    }
}

private static Class<?> getContainingClass(SerializedLambda lambda) {
    try {
        String className = lambda.getImplClass().replaceAll("/", ".");
        return Class.forName(className);
    } catch (Exception e) {
        throw new RuntimeException(e);
    }
}

private static Method method(Object lambda) {
    SerializedLambda serialized = serialized(lambda);
    Class<?> containingClass = getContainingClass(serialized);
    return Arrays.stream(containingClass.getDeclaredMethods())
                 .filter(method -> Objects.equals(method.getName(), serialized.getImplMethodName()))
                 .findFirst()
                 .orElseThrow(RuntimeException::new);
}

获取Java lambda表达式的封闭类

2 个答案: