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Whether you're building a web- or desktop-based application with Java SE or Java EE, many Java applications need to integrate data from a relational database. This course describes how to read and manage data from relational databases such as MySQL and SQL Server using the Java Database Connectivity (JDBC) API.
Author David Gassner explains how to choose a JDBC driver and connect to one or more databases. He also provides detailed instructions on reading, selecting, and updating data; calling stored procedures; managing data via JavaBean classes or with prepared statements; and working with metadata.
I have previously described how to use the database metadata class to get a list of tables from a database. I'm working in a new version of that project named TablesAndColumns where I already have the code to get the list of tables. I'll test it and make sure that I'm successfully retrieving that information. Now I'm going to show you how to get more information from the metadata object, including the names and data types of the database columns. I'll start by re-factoring this code. Instead of immediately outputting the name of each table, I'm going to store the names of the tables in an array list.
I'll start by declaring an array list at the top of the code, I will place my cursor before the try catch block, and I will declare my ArrayList, and I will set its data type to String, that is that's the data type of the items it will contain. And I will name it tables. I will instantiate it with the ArrayList class's no arguments constructor. Now I'll go down to the while loop, where I am looping through the result set of the tables. I'm still going to need the names of the tables, so I'll select that code and copy it to the clipboard, and then I'll delete that line.
Then I'll replace that code within the while loop with a call to the array list add method using tables.add, and I will paste in that code that was getting the table name already. So now I have a resizable array, an array list containing the names of the tables, and I can loop through that and get the columns for the tables one at a time. To contain that information, I'm going to need another result set. So I will move back up to where I'm declaring my local variables within the main method, and I'll declare another result set, this one called rsColumns, and I will initially set it to null.
Now I will move back to after the while loop, and I'm ready to process the tables. I will make some extra space. And to loop through the array list, I'll use a foreach code template. I'll type in foreach and press Ctrl+Space, and I'll choose that code template, and I will name the variable that's going to be generated each time through the loop, tableName. I'm going to be looping through the tables collection. So I will make that change as well. On each time through the table, I will first output the name of the table. I'm going to need three system output lines.
So I will type in System.out and press Ctrl+Space, and then I will duplicate that line a couple of times. For the first system output, I will output the name of the table prefixed with a label. I will type a label of Table: and a space, and I will append to that the tableName. The second output will be a string of dashes, that will separate the tableName from the columns, and the third one will just be a blank string, and I will output in the column names before that blank string, so I will make some extra space there and here is where we will put the code to get the list of the columns.
I have already declared my result set where I will contain the list of the columns. I named it rsColumns. So now I will get its reference. I will use rsColumns =, then just as I did for the tables, I will use a method of the metadata object. It will be metadata.getColumns. The getColumns method is very similar to the getTables method. It asks for four arguments. As with getTables, the catalog can be left to null because I have already designated the catalog or database I am working with in my connection string.
The schemaPattern can be set to a wildcard. The tableNamePattern should be the specific table that you are querying. So I'm going to pass in my variable tableName and the columnNamePattern can be a wildcard because I'm asking for all of the columns for the current table. So now my result set will contain all of the information I need about my columns. The result set will contain a bunch of columns, and you should take a look at the documentation for this method to see what the column names are, but the particular column names I'm interested in are called column_name and type_name.
I'm going to loop through the result set using a while loop with a condition. I will set my condition to a call to the rsColumn.next method. That will be rsColumns.next. Within the loop I will create a StringBuffer. I will name it buffer, and I will instantiate it with the no arguments constructor. I'll start by appending the name of the column, I'll use rsColumns.getString, and I will pass in the name of the column which will be COLUMN_NAME. I'll duplicate that line of code, and I'll change the second one from COLUMN_NAME to TYPE_NAME.
And in between, I will separate these bits of output with a colon and a space. Finally, I'll add some system output, and I'll output the value of the two string method of the buffer object. I will clean up the extra white space, and now I'm ready to test my application again. Let's review the code. I'm looping through the array list of tables. On each time through the loop I am outputting the name of the table and a separator string. Then I'm getting the list of columns just for that table and then I'm doing another loop, this time through the result set of columns.
I create a string buffer, and I append to that the name of the column, a separator and the name of the datatype and then I output that information. I'll move the cursor down to the finally block and do one last little bit of cleanup. I will close the rsColumns object, and now I'll run my code, and I get back exactly the list I am looking for. For each table, I'm displaying the name of the table and then after the separator the names and data types of each of the columns. As I have mentioned previously, look at the documentation for the getColumns method, you'll see a listing of all of the names of the columns that are returned in this result set.
You'll find that you can get the names of the columns, the lengths of the columns, and all of the other information you might need and look thoroughly at what else is available in the database metadata object, it will return information about stored procedures, functions, and pretty much everything that you have stored in your database.
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