csvreader_OpenCSV CSVReader CSVWriter示例

csvreader

OpenCSV is a lightweight java CSV parser. Today we will look into OpenCSV example for CSV parsing.

OpenCSV是轻量级的Java CSV解析器。今天，我们将研究用于CSV解析的OpenCSV示例。

OpenCSV (OpenCSV)

OpenCSV provides most of the basic features for CSV parsing. OpenCSV is more popular because we don’t have any builtin CSV parser in java. Some of the important classes in OpenCSV CSV parser are;

OpenCSV提供了CSV解析的大多数基本功能。 OpenCSV之所以受欢迎，是因为我们在Java中没有内置的CSV解析器。 OpenCSV CSV解析器中的一些重要类是：

CSVReader: This is the most important class in OpenCSV. CSVReader class is used to parse CSV files. We can parse CSV data line by line or read all data at once.CSVReader ：这是CSVReader最重要的类。 CSVReader类用于解析CSV文件。我们可以逐行解析CSV数据或一次读取所有数据。
CSVWriter: CSVWriter class is used to write CSV data to Writer implementation. You can define custom delimiter as well as quotes.CSVWriter ：CSVWriter类用于将CSV数据写入Writer实现。您可以定义自定义定界符以及引号。
CsvToBean: CsvToBean is used when you want to convert CSV data to java objects.CsvToBean ：当您要将CSV数据转换为Java对象时使用CsvToBean。
BeanToCsv: BeanToCsv is used to export Java beans to CSV file.BeanToCsv ：BeanToCsv用于将Java Bean导出到CSV文件。

OpenCSV Maven依赖关系 (OpenCSV Maven Dependency)

You can add OpenCSV jar using below maven dependency.

您可以使用以下maven依赖项添加OpenCSV jar。

<dependency><groupId>com.opencsv</groupId><artifactId>opencsv</artifactId><version>3.8</version>
</dependency>

Before we start looking at example program, we require demo CSV data and corresponding java bean.

在开始查看示例程序之前，我们需要演示CSV数据和相应的Java bean。

Here is our sample CSV file emps.csv

这是我们的示例CSV文件emps.csv

1,Pankaj Kumar,20,India
2,David Dan,40,USA
3,Lisa Ray,28,Germany

Below is our java bean class to hold CSV data.

以下是用于保存CSV数据的java bean类。

package com.journaldev.csv.model;public class Employee {private String id;private String name;private String age;private String country;public String getId() {return id;}public void setId(String id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}public String getAge() {return age;}public void setAge(String age) {this.age = age;}public String getCountry() {return country;}public void setCountry(String country) {this.country = country;}@Overridepublic String toString() {return "{" + id + "::" + name + "::" + age + "::" + country + "}";}
}

Let’s look at some common example of CSV parsing and CSV writing.

让我们看一些CSV解析和CSV编写的常见示例。

CSV阅读器 (CSVReader)

Our first CSVReader example is to read CSV file lines one by one and then convert to list of Employee.

我们的第一个CSVReader示例是一个接一个地读取CSV文件行，然后转换为Employee列表。

package com.journaldev.csv.opencsv.parser;import java.io.FileReader;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;import com.journaldev.csv.model.Employee;
import com.opencsv.CSVReader;/*** OpenCSV CSVReader Example, Read line by line* * @author pankaj**/
public class OpenCSVReaderLineByLineExample {public static void main(String[] args) throws IOException {CSVReader reader = new CSVReader(new FileReader("emps.csv"), ',');List<Employee> emps = new ArrayList<Employee>();// read line by lineString[] record = null;while ((record = reader.readNext()) != null) {Employee emp = new Employee();emp.setId(record[0]);emp.setName(record[1]);emp.setAge(record[2]);emp.setCountry(record[3]);emps.add(emp);}System.out.println(emps);reader.close();}}

Above CSVReader example is simple to understand. One important point is to close CSVReader to avoid memory leak. Also we can specify the delimiter character, just in case you are using something else.

上面的CSVReader示例很容易理解。重要的一点是关闭CSVReader以避免内存泄漏。同样，我们可以指定定界符，以防万一您使用其他字符。

Next CSVReader example is to read all the data in one shot using CSVReader readAll() method.

下一个CSVReader示例是使用CSVReader readAll()方法一次读取所有数据。

package com.journaldev.csv.opencsv.parser;import java.io.FileReader;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;import com.journaldev.csv.model.Employee;
import com.opencsv.CSVReader;/*** OpenCSV CSVReader Example, Read all at once* * @author pankaj**/
public class OpenCSVReaderReadAllExample {public static void main(String[] args) throws IOException {CSVReader reader = new CSVReader(new FileReader("emps.csv"), ',');List<Employee> emps = new ArrayList<Employee>();List<String[]> records = reader.readAll();Iterator<String[]> iterator = records.iterator();while (iterator.hasNext()) {String[] record = iterator.next();Employee emp = new Employee();emp.setId(record[0]);emp.setName(record[1]);emp.setAge(record[2]);emp.setCountry(record[3]);emps.add(emp);}System.out.println(emps);reader.close();}}

CsvToBean (CsvToBean)

We want to convert CSV to java object most of the time. We can use CsvToBean in these cases. Below is a simple example showing how to convert our employee CSV file to list of Employee objects.

我们希望大多数时间将CSV转换为java对象。在这些情况下，我们可以使用CsvToBean。下面是一个简单的示例，显示了如何将我们的员工CSV文件转换为Employee对象列表。

package com.journaldev.csv.opencsv.parser;import java.io.FileReader;
import java.io.IOException;
import java.util.List;import com.journaldev.csv.model.Employee;
import com.opencsv.CSVReader;
import com.opencsv.bean.ColumnPositionMappingStrategy;
import com.opencsv.bean.CsvToBean;
import com.opencsv.bean.HeaderColumnNameMappingStrategy;public class OpenCSVParseToBeanExample {public static void main(String[] args) throws IOException {CSVReader reader = new CSVReader(new FileReader("emps.csv"), ',');ColumnPositionMappingStrategy<Employee> beanStrategy = new ColumnPositionMappingStrategy<Employee>();beanStrategy.setType(Employee.class);beanStrategy.setColumnMapping(new String[] {"id","name","age","country"});CsvToBean<Employee> csvToBean = new CsvToBean<Employee>();List<Employee> emps = csvToBean.parse(beanStrategy, reader);System.out.println(emps);}
}

ColumnPositionMappingStrategy is used to map the CSV data row index to the Employee object fields.

ColumnPositionMappingStrategy用于将CSV数据行索引映射到Employee对象字段。

Sometimes our CSV file has header data too, for example we can have emps1.csv as below.

有时，我们的CSV文件也具有标头数据，例如，我们可以具有emps1.csv ，如下所示。

ID,NAME,age, country
1,Pankaj Kumar,20,India
2,David Dan,40,USA
3,Lisa Ray,28,Germany

In this case we can use HeaderColumnNameMappingStrategy as MappingStrategy implementation. Below is the method showing HeaderColumnNameMappingStrategy usage.

在这种情况下，我们可以将HeaderColumnNameMappingStrategy用作MappingStrategy实现。下面是显示HeaderColumnNameMappingStrategy用法的方法。

// returning list of Employee for CSVWriter example demo data
public static List<Employee> parseCSVWithHeader() throws IOException {CSVReader reader = new CSVReader(new FileReader("emps1.csv"), ',');HeaderColumnNameMappingStrategy<Employee> beanStrategy = new HeaderColumnNameMappingStrategy<Employee>();beanStrategy.setType(Employee.class);CsvToBean<Employee> csvToBean = new CsvToBean<Employee>();List<Employee> emps = csvToBean.parse(beanStrategy, reader);System.out.println(emps);reader.close();return emps;
}

CSVWriter (CSVWriter)

Let’s have a look at CSVWriter example to write java objects to CSV a Writer. We will reuse parseCSVWithHeader() defined above.

让我们看一下CSVWriter示例，该示例将Java对象写入CSV Writer。我们将重用上面定义的parseCSVWithHeader() 。

package com.journaldev.csv.opencsv.parser;import java.io.IOException;
import java.io.StringWriter;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;import com.journaldev.csv.model.Employee;
import com.opencsv.CSVWriter;public class OpenCSVWriterExample {public static void main(String[] args) throws IOException {StringWriter writer = new StringWriter();//using custom delimiter and quote characterCSVWriter csvWriter = new CSVWriter(writer, '#', '\'');List<Employee> emps = OpenCSVParseToBeanExample.parseCSVWithHeader();List<String[]> data = toStringArray(emps);csvWriter.writeAll(data);csvWriter.close();System.out.println(writer);}private static List<String[]> toStringArray(List<Employee> emps) {List<String[]> records = new ArrayList<String[]>();// adding header recordrecords.add(new String[] { "ID", "Name", "Age", "Country" });Iterator<Employee> it = emps.iterator();while (it.hasNext()) {Employee emp = it.next();records.add(new String[] { emp.getId(), emp.getName(), emp.getAge(), emp.getCountry() });}return records;}}

Notice the use of custom delimiter when writing theCSV data. We have also specified the quotes character to use with fields in CSV columns. Above CSVWriter example produces following output.

请注意，在写入CSV数据时使用了自定义分隔符。我们还指定了CSV列中的字段使用引号字符。上面的CSVWriter示例产生以下输出。

[{1::Pankaj Kumar::20::India}, {2::David Dan::40::USA}, {3::Lisa Ray::28::Germany}]
'ID'#'Name'#'Age'#'Country'
'1'#'Pankaj Kumar'#'20'#'India'
'2'#'David Dan'#'40'#'USA'
'3'#'Lisa Ray'#'28'#'Germany'

OpenCSV CSVWriter结果集 (OpenCSV CSVWriter ResultSet)

Sometimes we want to dump our database tables data to CSV files as backup. We can do that easily using CSVWriter writeAll(ResultSet rs, boolean includeColumnNames) method.

有时我们想将数据库表数据转储为CSV文件作为备份。我们可以使用CSVWriter writeAll(ResultSet rs, boolean includeColumnNames)方法轻松地做到这一点。

OpenCSV注释 (OpenCSV Annotation)

OpenCSV provides annotation based support too. Some of the OpenCSV annotations are;

OpenCSV也提供基于注释的支持。一些OpenCSV注释是；

CsvBindByName: for binding between a column name of the CSV input and a field in a bean.CsvBindByName ：用于在CSV输入的列名和bean中的字段之间进行绑定。
CsvBindByPosition: for binding between a column number of the CSV input and a field in a bean.CsvBindByPosition ：用于在CSV输入的列号和bean中的字段之间进行绑定。
CsvDate: for time based conversion.CsvDate ：用于基于时间的转换。

However I don’t want to use OpenCSV annotations because then my code will become tightly coupled with OpenCSV.

但是，我不想使用OpenCSV注释，因为那样我的代码将与OpenCSV紧密结合在一起。

That’s all for OpenCSV example tutorial.

这就是OpenCSV示例教程的全部内容。

Reference: OpenCSV Official Page

参考： OpenCSV官方页面

翻译自: https://www.journaldev.com/12014/opencsv-csvreader-csvwriter-example

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