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。
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");
}
}
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;
}