java - 使用不同的类加载器在 Java 中动态创建 lambda

Question

我正在尝试使用在编译时不可用的类动态创建 lambda 实例，因为它们是在运行时生成的，或者在编译时未知。

这可以使用以下代码

// lambdaClass = class of the lambda interface
// className = class containing the target method
// methodName = name of the target method
private static <T> T lookupLambda(Class<T> lambdaClass, String className, String methodName, Class<?> returnType,
          Class<?> argumentType) throws Throwable {
  MethodType lambdaType = MethodType.methodType(lambdaClass);
  MethodType methodType = MethodType.methodType(returnType, argumentType);
  MethodHandles.Lookup lookup = MethodHandles.lookup();
  Class<?> targetClass = Class.forName(className);
  MethodHandle handle = lookup.findStatic(targetClass, methodName, methodType);
  CallSite callSite = LambdaMetafactory
            .metafactory(lookup, "call", lambdaType, methodType.unwrap(), handle, methodType);
  MethodHandle methodHandle = callSite.getTarget();

  return lambdaClass.cast(methodHandle.invoke());
}

一个潜在的调用可能是这样的

@FunctionalInterface
interface MyLambda {
  double call(double d);
}

public void foo() {
  lookupLambda(MyLambda.class, "java.lang.Math", "sin", double.class, double.class);
}

在实验设置中，这很有效。然而，在实际代码中，lambda class 使用与应用程序其余部分不同的 ClassLoader 加载，即目标方法的 class。这会导致运行时出现异常，因为它似乎使用目标方法 class 的 ClassLoader 来加载 lambda class。这是堆栈跟踪中有趣的部分:

Caused by: java.lang.NoClassDefFoundError: GeneratedPackage.GeneratedClass$GeneratedInterface
    at sun.misc.Unsafe.defineAnonymousClass(Native Method)
    at java.lang.invoke.InnerClassLambdaMetafactory.spinInnerClass(InnerClassLambdaMetafactory.java:326)
    at java.lang.invoke.InnerClassLambdaMetafactory.buildCallSite(InnerClassLambdaMetafactory.java:194)
    at java.lang.invoke.LambdaMetafactory.metafactory(LambdaMetafactory.java:304)
    at my.project.MyClass.lookupLambda(MyClass.java:765)
    at 
    ... 9 more
Caused by: java.lang.ClassNotFoundException: GeneratedPackage.GeneratedClass$GeneratedInterface
    at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:335)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
    ... 15 more

我该如何解决这个问题？有没有办法指定每个类使用哪个 ClassLoader？是否有另一种动态创建不会遇到此问题的 lambda 实例的方法？非常感谢任何帮助。

编辑:这是一个应该显示问题的小可执行示例

<强>1。主类

import java.lang.invoke.CallSite;
import java.lang.invoke.LambdaMetafactory;
import java.lang.invoke.MethodHandle;
import java.lang.invoke.MethodHandles;
import java.lang.invoke.MethodType;
import java.lang.reflect.Constructor;
import java.lang.reflect.Method;
import java.net.URL;
import java.net.URLClassLoader;

public class Test {

  private static <T> T lookupLambda(Class<T> lambdaClass, String className, String methodName, Class<?> returnType,
      Class<?> argumentType) throws Throwable {
    MethodType lambdaType = MethodType.methodType(lambdaClass);
    MethodType methodType = MethodType.methodType(returnType, argumentType);
    MethodHandles.Lookup lookup = MethodHandles.lookup();
    Class<?> targetClass = Class.forName(className);
    MethodHandle handle = lookup.findStatic(targetClass, methodName, methodType);
    CallSite callSite = LambdaMetafactory
        .metafactory(lookup, "call", lambdaType, methodType.unwrap(), handle, methodType);
    MethodHandle methodHandle = callSite.getTarget();

    return lambdaClass.cast(methodHandle.invoke());
  }

  public static void main(String[] args) throws Throwable {
    URL resourcesUrl = new URL("file:/home/pathToGeneratedClassFile/");
    ClassLoader classLoader = new URLClassLoader(new URL[] { resourcesUrl }, Thread.currentThread().getContextClassLoader());

    Class<?> generatedClass = classLoader.loadClass("GeneratedClass");
    Class<?> generatedLambdaClass = classLoader.loadClass("GeneratedClass$GeneratedLambda");

    Constructor constructor = generatedClass.getConstructor(generatedLambdaClass);
    Object instance = constructor
        .newInstance(lookupLambda(generatedLambdaClass, "java.lang.Math", "sin", double.class, double.class));

    Method method = generatedClass.getDeclaredMethod("test");
    method.invoke(instance);
  }

}

<强>2。生成的类这假设该类已经被编译为 .class 文件，并且它位于系统类加载器范围之外的某个地方。

import javax.annotation.Generated;

@Generated("This class is generated and loaded using a different classloader")
public final class GeneratedClass {
  @FunctionalInterface
  public interface GeneratedLambda {
    double call(double d);
  }

  private final GeneratedLambda lambda;

  public GeneratedClass(GeneratedLambda lambda) {
    this.lambda = lambda;
  }

  public void test() {
    System.out.println(lambda.call(3));
  }

} 

对我来说，这会产生以下堆栈跟踪

Exception in thread "main" java.lang.NoClassDefFoundError: GeneratedClass$GeneratedLambda
    at sun.misc.Unsafe.defineAnonymousClass(Native Method)
    at java.lang.invoke.InnerClassLambdaMetafactory.spinInnerClass(InnerClassLambdaMetafactory.java:326)
    at java.lang.invoke.InnerClassLambdaMetafactory.buildCallSite(InnerClassLambdaMetafactory.java:194)
    at java.lang.invoke.LambdaMetafactory.metafactory(LambdaMetafactory.java:304)
    at Test.lookupLambda(Test.java:21)
    at Test.main(Test.java:36)
Caused by: java.lang.ClassNotFoundException: GeneratedClass$GeneratedLambda
    at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:335)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
    ... 6 more

Answer 1

我不知道你是如何创建你的类加载器的，但假设你已经有一个那么你可以替换

Class<?> targetClass = Class.forName(className);

与

Class<?> targetClass = yourClassLoader.loadClass(className);