我如何从Java 8 lambda内部抛出CHECKED异常,例如在流中使用?

换句话说,我想让代码像这样编译:

public List<Class> getClasses() throws ClassNotFoundException {     

    List<Class> classes = 
        Stream.of("java.lang.Object", "java.lang.Integer", "java.lang.String")
              .map(className -> Class.forName(className))
              .collect(Collectors.toList());                  
    return classes;
    }

这段代码无法编译,因为上面的Class.forName()方法会抛出ClassNotFoundException,该异常会被检查。

请注意,我不想将已检查异常包装在运行时异常中,并抛出已包装的未检查异常。我想抛出检查异常本身,而不向流添加丑陋的try/catch。


当前回答

在使用Stream时,我同意上面的评论。映射你被限制实现不抛出异常的函数。

然而,你可以创建自己的FunctionalInterface,抛出如下..

@FunctionalInterface
public interface UseInstance<T, X extends Throwable> {
  void accept(T instance) throws X;
}

然后使用Lambdas或引用实现它,如下所示。

import java.io.FileWriter;
import java.io.IOException;

//lambda expressions and the execute around method (EAM) pattern to
//manage resources

public class FileWriterEAM  {
  private final FileWriter writer;

  private FileWriterEAM(final String fileName) throws IOException {
    writer = new FileWriter(fileName);
  }
  private void close() throws IOException {
    System.out.println("close called automatically...");
    writer.close();
  }
  public void writeStuff(final String message) throws IOException {
    writer.write(message);
  }
  //...

  public static void use(final String fileName, final UseInstance<FileWriterEAM, IOException> block) throws IOException {

    final FileWriterEAM writerEAM = new FileWriterEAM(fileName);    
    try {
      block.accept(writerEAM);
    } finally {
      writerEAM.close();
    }
  }

  public static void main(final String[] args) throws IOException {

    FileWriterEAM.use("eam.txt", writerEAM -> writerEAM.writeStuff("sweet"));

    FileWriterEAM.use("eam2.txt", writerEAM -> {
        writerEAM.writeStuff("how");
        writerEAM.writeStuff("sweet");      
      });

    FileWriterEAM.use("eam3.txt", FileWriterEAM::writeIt);     

  }


 void writeIt() throws IOException{
     this.writeStuff("How ");
     this.writeStuff("sweet ");
     this.writeStuff("it is");

 }

}

其他回答

你不能。

然而,你可能想要看看我的一个项目,它可以让你更容易地操纵这种“投掷lambda”。

在你的情况下,你可以这样做:

import static com.github.fge.lambdas.functions.Functions.wrap;

final ThrowingFunction<String, Class<?>> f = wrap(Class::forName);

List<Class> classes =
    Stream.of("java.lang.Object", "java.lang.Integer", "java.lang.String")
          .map(f.orThrow(MyException.class))
          .collect(Collectors.toList());

并捕获MyException。

这是一个例子。另一个例子是你可以. orreturn()一些默认值。

请注意,这仍然是一项正在进行的工作,更多的是来。更好的名字,更多的功能等等。

在使用Stream时,我同意上面的评论。映射你被限制实现不抛出异常的函数。

然而,你可以创建自己的FunctionalInterface,抛出如下..

@FunctionalInterface
public interface UseInstance<T, X extends Throwable> {
  void accept(T instance) throws X;
}

然后使用Lambdas或引用实现它,如下所示。

import java.io.FileWriter;
import java.io.IOException;

//lambda expressions and the execute around method (EAM) pattern to
//manage resources

public class FileWriterEAM  {
  private final FileWriter writer;

  private FileWriterEAM(final String fileName) throws IOException {
    writer = new FileWriter(fileName);
  }
  private void close() throws IOException {
    System.out.println("close called automatically...");
    writer.close();
  }
  public void writeStuff(final String message) throws IOException {
    writer.write(message);
  }
  //...

  public static void use(final String fileName, final UseInstance<FileWriterEAM, IOException> block) throws IOException {

    final FileWriterEAM writerEAM = new FileWriterEAM(fileName);    
    try {
      block.accept(writerEAM);
    } finally {
      writerEAM.close();
    }
  }

  public static void main(final String[] args) throws IOException {

    FileWriterEAM.use("eam.txt", writerEAM -> writerEAM.writeStuff("sweet"));

    FileWriterEAM.use("eam2.txt", writerEAM -> {
        writerEAM.writeStuff("how");
        writerEAM.writeStuff("sweet");      
      });

    FileWriterEAM.use("eam3.txt", FileWriterEAM::writeIt);     

  }


 void writeIt() throws IOException{
     this.writeStuff("How ");
     this.writeStuff("sweet ");
     this.writeStuff("it is");

 }

}

只要使用Lombok的@SneakyThrows就可以了。

Christian Hujer已经详细解释了为什么严格来说,由于Java的限制,从流中抛出受控异常是不可能的。

其他一些回答解释了一些技巧,可以绕过语言的限制,但仍然能够满足抛出“检查异常本身,并且不向流中添加丑陋的try/catch”的要求,其中一些需要额外的数十行样板文件。

我要强调的是另一个选项,在我看来,这个选项比其他选项干净得多:Lombok的@SneakyThrows。这个问题在其他答案中也提到过,但有很多不必要的细节。

生成的代码非常简单:

public List<Class> getClasses() throws ClassNotFoundException {
    List<Class> classes =
        Stream.of("java.lang.Object", "java.lang.Integer", "java.lang.String")
                .map(className -> getClass(className))
                .collect(Collectors.toList());
    return classes;
}

@SneakyThrows                                 // <= this is the only new code
private Class<?> getClass(String className) {
    return Class.forName(className);
}

我们只需要一个提取方法重构(由IDE完成)和一行额外的@SneakyThrows。注释负责添加所有样板文件,以确保您可以抛出检查过的异常,而无需将其包装在RuntimeException中,也无需显式声明它。

你可以用apache commons-lang3库来实现。

https://commons.apache.org/proper/commons-lang/javadocs/api-release/org/apache/commons/lang3/function/Failable.html

public List<Class> getClasses() throws ClassNotFoundException {
    List<Class> classes =
            Stream.of("java.lang.Object", "java.lang.Integer", "java.lang.String")
                    .map(Failable.asFunction(Class::forName))
                    .collect(Collectors.toList());
    return classes;
}

这个答案类似于17,但避免了包装器异常定义:

List test = new ArrayList();
        try {
            test.forEach(obj -> {

                //let say some functionality throws an exception
                try {
                    throw new IOException("test");
                }
                catch(Exception e) {
                    throw new RuntimeException(e);
                }
            });
        }
        catch (RuntimeException re) {
            if(re.getCause() instanceof IOException) {
                //do your logic for catching checked
            }
            else 
                throw re; // it might be that there is real runtime exception
        }