如何在Java中将CLOB类型转换为String？
CLOB通常代表CharacterLargeObject，SQLClob是内置数据类型，用于存储大量文本数据。使用此数据类型，您最多可以存储2,147,483,647个字符。
JDBCAPI的java.sql.Clob接口表示CLOB数据类型。由于JDBC中的Clob对象是使用SQL定位器实现的，因此它包含指向SQLCLOB（而不是数据）的逻辑指针。
MySQL数据库使用四个变量TINYTEXT，TEXT，MEDIUMTEXT和LONGTEXT为该数据类型提供支持。
要将CLOB数据类型转换为字符串
使用PresparedStatement接口的getClob()或getCharacterStream()方法从表中检索Clob值。
Reader r = clob.getCharacterStream();
从检索到的字符流中逐个读取每个字符，并将它们附加到StringBuilder或StringBuffer。
int j = 0;
StringBuffer buffer = new StringBuffer();
int ch;
while ((ch = r.read())!=-1) {
buffer.append(""+(char)ch);
}
System.out.println(buffer.toString());
j++;最后，显示或存储获得的String。
System.out.println(buffer.toString());
示例
让我们使用以下查询在MySQL数据库中创建一个名称为Technologies_data的表-
CREATE TABLE Technologies (Name VARCHAR(255), Type VARCHAR(255), Article LONGTEXT);
表Article的第三列存储CLOB类型的数据。
接下来的JDBC程序最初在technologies_data表中插入5条记录，将文本文件（其内容）存储到article列（CLOB类型）。
然后，它检索表的记录，并显示文章的名称和内容。在这里，我们试图将检索到的CLOB的数据转换为String并显示它。
import java.io.FileReader;
import java.io.Reader;
import java.sql.Clob;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.Statement;
public class ClobToString {
public static void main(String args[]) throws Exception {
//Registering the Driver
DriverManager.registerDriver(new com.mysql.jdbc.Driver());
//Getting the connection
String mysqlUrl = "jdbc:mysql://localhost/sampledatabase";
Connection con = DriverManager.getConnection(mysqlUrl, "root", "password");
System.out.println("Connection established......");
//Creating a Statement object
Statement stmt = con.createStatement();
//Inserting values
String query = "INSERT INTO Technologies_data VALUES (?, ?, ?)";
PreparedStatement pstmt = con.prepareStatement(query);
pstmt.setString(1, "JavaFX");
pstmt.setString(2, "Java Library");
FileReader reader = new FileReader("E:\\images\\javafx_contents.txt");
pstmt.setClob(3, reader);
pstmt.execute();
pstmt.setString(1, "CoffeeScript");
pstmt.setString(2, "Scripting Language");
reader = new FileReader("E:\\images\\coffeescript_contents.txt");
pstmt.setClob(3, reader);
pstmt.execute();
pstmt.setString(1, "Cassandra");
pstmt.setString(2, "NoSQL Database");
reader = new FileReader("E:\\images\\cassandra_contents.txt");
pstmt.setClob(3, reader);
pstmt.execute();
//Retrieving the data
ResultSet rs = stmt.executeQuery("select * from Technologies_data");
System.out.println("Contents of the table are: ");
while(rs.next()) {
System.out.println("Article: "+rs.getString("Name"));
Clob clob = rs.getClob("Article");
Reader r = clob.getCharacterStream();
StringBuffer buffer = new StringBuffer();
int ch;
while ((ch = r.read())!=-1) {
buffer.append(""+(char)ch);
}
System.out.println("Contents: "+buffer.toString());
System.out.println(" ");
}
}
}输出结果
Connection established...... Contents of the table are: Article: JavaFX Contents: JavaFX is a Java library using which you can develop Rich Internet Applications. By using Java technology, these applications have a browser penetration rate of 76%. Article: CoffeeScript Contents: CoffeeScript is a lightweight language based on Ruby and Python which transcompiles (compiles from one source language to another) into JavaScript. It provides better syntax avoiding the quirky parts of JavaScript, still retaining the flexibility and beauty of the language. Article: Cassandra Contents: Apache Cassandra is a highly scalable, high-performance distributed database designed to handle large amounts of data across many commodity servers, providing high availability with no single point of failure. It is a type of NoSQL database. Let us first understand what a NoSQL database does.