Apache Commons CSV介绍

1. Overview

Apache Commons CSV library has many useful features for creating and reading CSV files.

In this quick tutorial, we’ll see how to utilize this library by showing a simple example.

2. Maven Dependency

To start, we will import the latest version of this library using Maven:

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-csv</artifactId>
    <version>1.10.0</version>
</dependency>

To check for the most recent version of this library – go here.

3. Reading a CSV File

Consider the following CSV file called book.csv containing the attributes of a book:

author,title
Dan Simmons,Hyperion
Douglas Adams,The Hitchhiker's Guide to the Galaxy

Let’s see how we can read it:

Map<String, String> AUTHOR_BOOK_MAP = new HashMap<>() {
    {
        put("Dan Simmons", "Hyperion");
        put("Douglas Adams", "The Hitchhiker's Guide to the Galaxy");
    }
});
String[] HEADERS = { "author", "title"};

@Test
void givenCSVFile_whenRead_thenContentsAsExpected() throws IOException {
    Reader in = new FileReader("src/test/resources/book.csv");

    CSVFormat csvFormat = CSVFormat.DEFAULT.builder()
        .setHeader(HEADERS)
        .setSkipHeaderRecord(true)
        .build();

    Iterable<CSVRecord> records = csvFormat.parse(in);

    for (CSVRecord record : records) {
        String author = record.get("author");
        String title = record.get("title");
        assertEquals(AUTHOR_BOOK_MAP.get(author), title);
    }
}

We are reading the records of a CSV file after skipping the first line as it is the header.

There are different types of CSVFormat specifying the format of the CSV file, an example of which you can see in the next paragraph.

4. Creating a CSV File

Let’s see how we can create the same CSV file as above:

@Test
void givenAuthorBookMap_whenWrittenToStream_thenOutputStreamAsExpected() throws IOException {
    StringWriter sw = new StringWriter();

    CSVFormat csvFormat = CSVFormat.DEFAULT.builder()
        .setHeader(HEADERS)
        .build();
    
    try (final CSVPrinter printer = new CSVPrinter(sw, csvFormat)) {
        AUTHOR_BOOK_MAP.forEach((author, title) -> {
            try {
                printer.printRecord(author, title);
            } catch (IOException e) {
                e.printStackTrace();
            }
        });
    }
    assertEquals(EXPECTED_FILESTREAM, sw.toString().trim());
}

The new CSV file will be created with the appropriate headers because we have specified them in our CSVFormat declaration.

5. Headers & Reading Columns

There are different ways to read and write headers. Similarly, there are different ways to read column values.

Let’s go through them one by one:

5.1. Accessing Columns by Index

This is the most basic way to read column values. This can be used when the headers for the CSV files are not known:

Reader in = new FileReader("book.csv");
Iterable<CSVRecord> records = csvFormat.parse(in);
for (CSVRecord record : records) {
    String columnOne = record.get(0);
    String columnTwo = record.get(1);
}

5.2. Accessing Columns by Predefined Headers

This is a more intuitive way to access columns when compared to accessing by indices:

Iterable<CSVRecord> records = csvFormat.parse(in);
for (CSVRecord record : records) {
    String author = record.get("author");
    String title = record.get("title");
}

5.3. Using Enums as Headers

Using Strings for accessing column values can be error-prone. Using Enums instead of Strings will make the code more standardized and easier to understand:

enum BookHeaders{
    author, title
}    

CSVFormat csvFormat = CSVFormat.DEFAULT.builder()
    .setHeader(BookHeaders.class)
    .setSkipHeaderRecord(true)
    .build();

Iterable<CSVRecord> records = csvFormat.parse(in);

for (CSVRecord record : records) {
    String author = record.get(BookHeaders.author);
    String title = record.get(BookHeaders.title);
    assertEquals(AUTHOR_BOOK_MAP.get(author), title);
}

5.4. Skipping the Header Line

Usually, CSV files contain headers in the first line. Hence, in most cases, it is safe to skip it and start reading from the second row.

This will autodetect headers access column values:

CSVFormat csvFormat = CSVFormat.DEFAULT.builder()
    .setSkipHeaderRecord(true)
    .build();

Iterable<CSVRecord> records = csvFormat.parse(in);

for (CSVRecord record : records) {
    String author = record.get("author");
    String title = record.get("title");
}

5.5. Creating a File With Headers

Similarly, we can create a CSV file with the first line containing the headers:

FileWriter out = new FileWriter("book_new.csv");
CSVPrinter printer = csvFormat.print(out);

6. Conclusion

We presented the use of Apache’s Commons CSV library through a simple example. You can read more about the library here.

The code for this article is available over on Github.