我有以下程序:
workingPopulateFromFileAsStream
,我已经将其再次明确地转换为流,然后可以正常工作。student.txt的内容:
Name101,Last101
Name102,Last102
Name103,Last103
Name104,Last104
Name105,Last105
Name106,Last106
Name107,Last107
Name108,Last108
Name109,Last109
Name110,Last110
ClientMainApp.java:
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.*;
import java.util.stream.Collectors;
import java.util.stream.Stream;
class Student {
String first_name;
String last_name;
public Student(String first_name, String last_name) {
this.first_name = first_name;
this.last_name = last_name;
}
public String toString() {
return this.first_name + " " + this.last_name;
}
}
public class ClientMainApp {
public static void main(String... args) {
String fileName = "student.txt";
ClientMainApp obj = new ClientMainApp();
//Working Code.
Stream<Student> students_stream = obj.workingPopulateFromFileAsStream(fileName);
System.out.println("working: Got Student Stream as " + students_stream);
students_stream.forEach(System.out::println);
System.out.println("==================================================");
// Not Working Code.
Stream<Student> students_stream2 = obj.populateFromFileAsStream2(fileName);
System.out.println("notWorking: Got Student Stream2 as " + students_stream2);
students_stream2.forEach(System.out::println);
}
private Stream<Student> workingPopulateFromFileAsStream(String fileName) {
try (Stream<String> stream = Files.lines(Paths.get(fileName))) {
System.out.println("Working: Got File Stream as " + stream);
Stream<Student> retval =
stream
.map(
st -> {
ArrayList<String> record = new ArrayList<>(Arrays.asList(st.split(",")));
return new Student(record.get(0), record.get(1));
})
.collect(Collectors.toList()).stream(); // This is the code making the difference.
System.out.println("Working: Got Retval Stream as " + retval);
return retval;
} catch (IOException e) {
e.printStackTrace();
}
return null;
}
private Stream<Student> populateFromFileAsStream2(String fileName) {
try (Stream<String> stream = Files.lines(Paths.get(fileName))) {
System.out.println("NotWorking: Got File Stream as " + stream);
Stream<Student> retval =
stream.map(
st -> {
ArrayList<String> record = new ArrayList<>(Arrays.asList(st.split(",")));
return new Student(record.get(0), record.get(1));
});
System.out.println("NotWorking: got Retval Stream as " + retval);
return retval;
} catch (IOException e) {
e.printStackTrace();
}
return null;
}
}
我得到的结果是:
Working: Got File Stream as java.util.stream.ReferencePipeline$Head@3e3abc88
Working: Got Retval Stream as java.util.stream.ReferencePipeline$Head@214c265e
working: Got Student Stream as java.util.stream.ReferencePipeline$Head@214c265e
Name101 Last101
Name102 Last102
Name103 Last103
Name104 Last104
Name105 Last105
Name106 Last106
Name107 Last107
Name108 Last108
Name109 Last109
Name110 Last110
==================================================
NotWorking: Got File Stream as java.util.stream.ReferencePipeline$Head@7699a589
NotWorking: got Retval Stream as java.util.stream.ReferencePipeline$3@4dd8dc3
notWorking: Got Student Stream2 as java.util.stream.ReferencePipeline$3@4dd8dc3
Exception in thread "main" java.lang.IllegalStateException: source already consumed or closed
at java.util.stream.AbstractPipeline.sourceSpliterator(AbstractPipeline.java:406)
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
at java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:418)
at ClientMainApp.main(ClientMainApp.java:38)
在响应中,我们可以看到retVal的流对象是相同的,并且返回给调用函数,但是我不确定为什么它说它已经被消耗了。
[请注意,如果我在populateFromFileAsStream2
内使用forEach循环,则可以正常工作。仅当从函数返回值时,才会给出错误。
您的非工作版本本质上是这样的:
public Stream<String> foo(Path file) throws IOException {
try (Stream<String> stream = Files.lines(file)) {
return stream.map(...);
}
}
您正在使用try-with-resources statement。这意味着,一旦退出Stream
块,就将try
关闭,必须执行此操作才能使方法返回。因此,您要在从方法返回流之前关闭流。也就是说,我认为您的错误根源于:
- 使用上述流的stream.map()创建一个新流,并将其存储在retVal中。
尽管确实是map
将返回一个新的Stream
对象,但新对象仍然是同一概念流/管道的一部分。换句话说,关闭stream
参考与关闭retVal
参考相同。您的工作代码通过返回一个全新的管道来解决此问题,尽管以将整个文件缓冲到内存中为代价。