使用Observable和take运算符读取文件的RxJava问题

Question

...
Observable<String> fileLineObservable = Observable.defer(new Callable<String>(){
    return new ObservableSource<String> call() throws Exception {
        return new ObservableSource<String>() {
            public void subscribe(Observer<String> observer) {
                BufferedReader reader = null;
                try {
                    reader = new BufferedReader(new FileReader(filePath));
                    String line = null;
                    while ((line = reader.readLine()) != null) {
                        observer.onNext(line);
                    }
                    observer.onComplete();

                    ... catching exception and close reader
                }
            }
        }
    }
});

我还想用一个take（count）运算符使一个观察上述情况的观察者成为可观察的，如下所示：

fileLineObservable.take(2)
                  .subscribe(new Consumer<String>() {
                       public void onNext(String line) {
                           ... do something with the file line string
                       }
                  });

我在执行上述代码时遇到NullPointerException,我知道原因。NPE是由于onNext的第二次调用导致在TakeObserver实例上执行onComplete，并且在onComplete方法内部调用未设置（null）的upStream.Dispose。TakeObserver的上游变量在订阅一个Observable时应该设置为onSubscribe（Disposable Disposable）。

我该如何解决这个问题呢？我是否应该实现我自己的一次性类来设置TakeObserver的上游？

Answer 1

这个解决方案呢？

Observable<String> observableFile2(Path path) {
        return Observable.using(
                () -> Files.newBufferedReader(path),
                reader -> {
                    return Observable.fromIterable(() -> {
                        return new Iterator<>() {
                            private String nextLine = null;

                            @Override
                            public boolean hasNext() {
                                try {
                                    nextLine = reader.readLine();
                                    return nextLine != null;
                                } catch (Exception ex) {
                                    return false;
                                }
                            }

                            @Override
                            public String next() {
                                if (nextLine != null) {
                                    return nextLine;
                                }
                                throw new IllegalStateException("nextLine can not be null.");
                            }
                        };
                    });
                },
                BufferedReader::close
        );
    }

• Observable#using确保在disposable/OnError上正确关闭BufferedReader
• Observable#fromiterable包装readLine调用并为我们处理onComplete。

测试

testImplementation("org.junit.jupiter:junit-jupiter-api:5.6.2")
testRuntimeOnly("org.junit.platform:junit-platform-launcher:1.6.2")
testRuntimeOnly("org.junit.jupiter:junit-jupiter-engine:5.6.2")
testRuntimeOnly("org.junit.vintage:junit-vintage-engine:5.6.2")
testImplementation("com.google.jimfs:jimfs:1.1")

@Test
void name() {
    observableFile2(hello).take(2)
            .test()
            .assertValues("line0", "line1")
            .assertComplete();
}

@Test
void name2() {
    observableFile2(hello).take(10)
            .test()
            .assertValues("line0", "line1", "line2", "line3")
            .assertComplete();
}

@Test
void name3() {
    observableFile2(hello2)
            .test()
            .assertComplete();
}