Java 使用回调函数将字符串追加到文件
我正在从数据库中读取数据,并且喜欢使用回调来追加数据库中的每一行数据。我已经设法使回调正常工作,但我不知道如何将数据附加到文件中。这是我的密码 主要 菲利奥Java 使用回调函数将字符串追加到文件,java,Java,我正在从数据库中读取数据,并且喜欢使用回调来追加数据库中的每一行数据。我已经设法使回调正常工作,但我不知道如何将数据附加到文件中。这是我的密码 主要 菲利奥 public class FileIO implements DBAccess.CallBack { public void writeRStoFile() { DBAccess dbAccess = new DBAccess(this); String fileName = "/result.
public class FileIO implements DBAccess.CallBack {
public void writeRStoFile() {
DBAccess dbAccess = new DBAccess(this);
String fileName = "/result.csv";
try (FileWriter fw = new FileWriter(fileName)) {
System.out.println("Starting data download from DB...");
dbAccess.readDB();
// HERE I LIKE TO APPEND EACH ROW TO THE FILE
fw.append('\n');
System.out.println("Finished data download from DB...");
} catch (IOException e) {
e.printStackTrace();
}
}
@Override
public void iAmDone(String row) {
System.out.println("Row: " + row);
}
}
数据库访问
public class DBAccess {
public interface CallBack {
public void iAmDone(String row);
}
private final CallBack callBack;
public DBAccess(CallBack callBack) {
this.callBack = callBack;
}
public void readDB() {
String url = "jdbc:Cobol:////Dev/Project Files/DatAndCpyFiles";
try (Connection con = DriverManager.getConnection(url, "", "");
Statement stmt = con.createStatement())
{
stmt.setFetchSize(10);
Class.forName("com.hxtt.sql.cobol.CobolDriver").newInstance();
String sql = "select * from PROFS";
ResultSet rs = stmt.executeQuery(sql);
ResultSetMetaData resultSetMetaData = rs.getMetaData();
int iNumCols = resultSetMetaData.getColumnCount();
for (int i = 1; i <= iNumCols; i++) {
callBack.iAmDone(resultSetMetaData.getColumnLabel(i) + ";");
}
String field;
while (rs.next()) {
String row = "";
for (int i = 1; i <= iNumCols; i++) {
field = rs.getString(i);
field = field.trim() + ";";
row = row + field;
}
callBack.iAmDone(row);
}
rs.close();
} catch (Exception e) {
System.out.println(e.getMessage());
e.printStackTrace();
}
}
}
公共类DBAccess{
公共接口回调{
已完成的公共作废(字符串行);
}
私有最终回调;
公共数据库访问(回调){
this.callBack=回调;
}
public void readDB(){
String url=“jdbc:Cobol:////Dev/Project 文件/DatAndCpyFiles”;
try(Connection con=DriverManager.getConnection(url,“,”);
语句stmt=con.createStatement())
{
stmt.setFetchSize(10);
Class.forName(“com.hxtt.sql.cobol.CobolDriver”).newInstance();
String sql=“从PROFS中选择*”;
结果集rs=stmt.executeQuery(sql);
ResultSetMetaData ResultSetMetaData=rs.getMetaData();
int iNumCols=resultSetMetaData.getColumnCount();
对于(int i=1;i我可以想到的一种方法是在FileIO
中将fw
声明为成员变量。这样,您就可以在iAmDone
中调用fw.append(…)
(您可能需要更改try with resources)问题不在于追加。问题在于将iAmDone
链接到fw.append
。
public class DBAccess {
public interface CallBack {
public void iAmDone(String row);
}
private final CallBack callBack;
public DBAccess(CallBack callBack) {
this.callBack = callBack;
}
public void readDB() {
String url = "jdbc:Cobol:////Dev/Project Files/DatAndCpyFiles";
try (Connection con = DriverManager.getConnection(url, "", "");
Statement stmt = con.createStatement())
{
stmt.setFetchSize(10);
Class.forName("com.hxtt.sql.cobol.CobolDriver").newInstance();
String sql = "select * from PROFS";
ResultSet rs = stmt.executeQuery(sql);
ResultSetMetaData resultSetMetaData = rs.getMetaData();
int iNumCols = resultSetMetaData.getColumnCount();
for (int i = 1; i <= iNumCols; i++) {
callBack.iAmDone(resultSetMetaData.getColumnLabel(i) + ";");
}
String field;
while (rs.next()) {
String row = "";
for (int i = 1; i <= iNumCols; i++) {
field = rs.getString(i);
field = field.trim() + ";";
row = row + field;
}
callBack.iAmDone(row);
}
rs.close();
} catch (Exception e) {
System.out.println(e.getMessage());
e.printStackTrace();
}
}
}