如何从 JDBC ResultSet 获取列数？

You can get columns number from ResultSetMetaData:

Statement st = conn.createStatement();
ResultSet rs = st.executeQuery(query);
ResultSetMetaData rsmd = rs.getMetaData();


int columnsNumber = rsmd.getColumnCount();

PreparedStatement ps=con.prepareStatement("select * from stud");


ResultSet rs=ps.executeQuery();


ResultSetMetaData rsmd=rs.getMetaData();


System.out.println("columns: "+rsmd.getColumnCount());
System.out.println("Column Name of 1st column: "+rsmd.getColumnName(1));
System.out.println("Column Type Name of 1st column: "+rsmd.getColumnTypeName(1));

Number of a columns in the result set you can get with code (as DB is used PostgreSQL):

//load the driver for PostgreSQL
Class.forName("org.postgresql.Driver");


String url = "jdbc:postgresql://localhost/test";
Properties props = new Properties();
props.setProperty("user","mydbuser");
props.setProperty("password","mydbpass");
Connection conn = DriverManager.getConnection(url, props);


//create statement
Statement stat = conn.createStatement();


//obtain a result set
ResultSet rs = stat.executeQuery("SELECT c1, c2, c3, c4, c5 FROM MY_TABLE");


//from result set give metadata
ResultSetMetaData rsmd = rs.getMetaData();


//columns count from metadata object
int numOfCols = rsmd.getColumnCount();

But you can get more meta-informations about columns:

for(int i = 1; i <= numOfCols; i++)
{
System.out.println(rsmd.getColumnName(i));
}

And at least but not least, you can get some info not just about table but about DB too, how to do it you can find here and here.

After establising the connection and executing the query try this:

 ResultSet resultSet;
int columnCount = resultSet.getMetaData().getColumnCount();
System.out.println("column count : "+columnCount);

This will print the data in columns and comes to new line once last column is reached.

ResultSetMetaData resultSetMetaData = res.getMetaData();
int columnCount = resultSetMetaData.getColumnCount();
for(int i =1; i<=columnCount; i++){
if(!(i==columnCount)){


System.out.print(res.getString(i)+"\t");
}
else{
System.out.println(res.getString(i));
}


}