Chapter 4. Writing to Entity Stores
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## Chapter 4. Writing to Entity Stores

Once you have implemented your classes for storage using the DPL, then writing your instances to the entity store is straight-forward. Simply open the `EntityStore` and then put the instance to it using the appropriate primary index (which you retrieve from the entity store).

There are a few caveats that you should consider before we proceed.

First, if you have not provided a value to the primary key field for the object you are storing, then you will see runtime errors when you try to write the object to the store. The primary key is not allowed to be null.

As described in Vendor.class, you can avoid the need to explicitly set a primary key by declaring a sequence for the primary key. In our example, however, we explicitly set a primary key value.

Secondly, you need to decide if you want to use transactions to write to the entity store. Transactions are a mechanism that allow you to group multiple write operations in a single atomic unit; either all the operations succeed or none of them succeed. Transactions also provide isolation guarantees for multi-threaded applications.

Transactions represent a large topic that are described in the Berkeley DB Java Edition Getting Started with Transaction Processing guide. For our purposes here, it is important to know the following:

- Once you open an `EntityStore` as transactional, all write operations are performed using a transaction whether or not you explicitly use transactions. If you do not explicitly use a transaction, then each individual write operation is performed under a transaction using auto-commit.

- If you explicitly use a transaction, then you must either commit or abort the transaction when you are done performing write operations for that transaction instance.

All of these topics, and more, are described in detail in the Transaction Processing guide noted above.

Because it is likely that your application will use transactions, we show their usage here in a minimal way.

## ExampleDatabasePut.class

Our example reads inventory and vendor information from flat text files, encapsulates this data in objects of the appropriate type, and then writes each object to an `EntityStore`.

`Vendor` objects are written one at a time without use of an explicit transaction handle. Because the store is opened for transactional writes, this means that each such write operation is performed under a transaction using auto-commit.

`Inventory` class objects are all written using a single transaction. This means that if any failures are encountered during the data write, the entire operation is aborted and the store will have no inventory information in it.

There only reason why we use transactions in this way is for illustrative purposes. You could just as easily use auto-commit for all the operations, or wrap both the vendor and the inventory writes in a single giant transaction, or do something in between.

To begin, we import the Java classes that our example needs. Most of the imports are related to reading the raw data from flat text files and breaking them apart for usage with our data classes. We also import classes from the JE package, but we do not actually import any classes from the DPL. The reason why is because we have placed almost all of our DPL work off into other classes, so there is no need for direct usage of those APIs here.

    package persist.gettingStarted;

    import java.io.BufferedReader;
    import java.io.File;
    import java.io.FileInputStream;
    import java.io.FileNotFoundException;
    import java.io.IOException;
    import java.io.InputStreamReader;
    import java.util.ArrayList;
    import java.util.List;

    import com.sleepycat.je.DatabaseException;
    import com.sleepycat.je.Transaction;

Now we can begin the class itself. Here we set default paths for the on-disk resources that we require (the environment home, and the location of the text files containing our sample data). We also declare `DataAccessor` and `MyDbEnv` members. We describe these classes and show their implementation in DataAccessor.class and MyDbEnv.

    public class ExampleDatabasePut {

        private static File myDbEnvPath = new File("/tmp/JEDB");
        private static File inventoryFile = new File("./inventory.txt");
        private static File vendorsFile = new File("./vendors.txt");

        private DataAccessor da;

        // Encapsulates the environment and data store.
        private static MyDbEnv myDbEnv = new MyDbEnv();

Next, we provide our `usage()` method. The command line options provided there are necessary only if the default values to the on-disk resources are not sufficient.

        private static void usage() {
            System.out.println("ExampleDatabasePut [-h <env directory>]");
            System.out.println("      [-i <inventory file>] [-v <vendors file>]");
            System.exit(-1);
        }

Our `main()` method is also reasonably self-explanatory. We simply instantiate an `ExampleDatabasePut` object there and then call it's `run()` method. We also provide a top-level `try` block there for any exceptions that might be thrown during runtime.

Notice that the `finally` statement in the top-level `try` block calls `MyDbEnv.close()`. This method closes our `EntityStore` and `Environment` objects. By placing it here in the `finally` statement, we can make sure that our store and environment are always cleanly closed.

        public static void main(String args[]) {
            ExampleDatabasePut edp = new ExampleDatabasePut();
            try {
                edp.run(args);
            } catch (DatabaseException dbe) {
                System.err.println("ExampleDatabasePut: " + dbe.toString());
                dbe.printStackTrace();
            } catch (Exception e) {
                System.out.println("Exception: " + e.toString());
                e.printStackTrace();
            } finally {
                myDbEnv.close();
            }
            System.out.println("All done.");
        }

Our `run()` method does four things. It calls `MyDbEnv.setup()`, which opens our `Environment` and `EntityStore`. It then instantiates a `DataAccessor` object, which we will use to write data to the store. It calls `loadVendorsDb()` which loads all of the vendor information. And then it calls `loadInventoryDb()` which loads all of the inventory information.

Notice that the `MyDbEnv` object is being setup as read-write. This results in the `EntityStore` being opened for transactional support. (See MyDbEnv for implementation details.)

        private void run(String args[])
            throws DatabaseException {
            // Parse the arguments list
            parseArgs(args);

            myDbEnv.setup(myDbEnvPath,  // Path to the environment home
                          false);       // Environment read-only?

            // Open the data accessor. This is used to store
            // persistent objects.
            da = new DataAccessor(myDbEnv.getEntityStore());

            System.out.println("loading vendors db....");
            loadVendorsDb();

            System.out.println("loading inventory db....");
            loadInventoryDb();
        }

We can now implement the `loadVendorsDb()` method. This method is responsible for reading the vendor contact information from the appropriate flat-text file, populating `Vendor` class objects with the data and then writing it to the `EntityStore`. As explained above, each individual object is written with transactional support. However, because a transaction handle is not explicitly used, the write is performed using auto-commit. This happens because the `EntityStore` was opened to support transactions.

To actually write each class to the `EntityStore`, we simply call the `PrimaryIndex.put()` method for the `Vendor` entity instance. We obtain this method from our `DataAccessor` class.

        private void loadVendorsDb()
                throws DatabaseException {

            // loadFile opens a flat-text file that contains our data
            // and loads it into a list for us to work with. The integer
            // parameter represents the number of fields expected in the
            // file.
            List vendors = loadFile(vendorsFile, 8);

            // Now load the data into the store.
            for (int i = 0; i < vendors.size(); i++) {
                String[] sArray = (String[])vendors.get(i);
                Vendor theVendor = new Vendor();
                theVendor.setVendorName(sArray[0]);
                theVendor.setAddress(sArray[1]);
                theVendor.setCity(sArray[2]);
                theVendor.setState(sArray[3]);
                theVendor.setZipcode(sArray[4]);
                theVendor.setBusinessPhoneNumber(sArray[5]);
                theVendor.setRepName(sArray[6]);
                theVendor.setRepPhoneNumber(sArray[7]);

                // Put it in the store. Because we do not explicitly set
                // a transaction here, and because the store was opened
                // with transactional support, auto commit is used for each
                // write to the store.
                da.vendorByName.put(theVendor);
            }
        }

Now we can implement our `loadInventoryDb()` method. This does exactly the same thing as the `loadVendorsDb()` method except that the entire contents of the inventory data is loaded using a single transaction.

Again, the transactional activity shown here is described in detail in the Berkeley DB Java Edition Getting Started with Transaction Processing guide.

        private void loadInventoryDb()
            throws DatabaseException {

            // loadFile opens a flat-text file that contains our data
            // and loads it into a list for us to work with. The integer
            // parameter represents the number of fields expected in the
            // file.
            List inventoryArray = loadFile(inventoryFile, 6);

            // Now load the data into the store. The item's sku is the
            // key, and the data is an Inventory class object.

            // Start a transaction. All inventory items get loaded using a
            // single transaction.
            Transaction txn = myDbEnv.getEnv().beginTransaction(null, null);

            for (int i = 0; i < inventoryArray.size(); i++) {
                String[] sArray = (String[])inventoryArray.get(i);
                String sku = sArray[1];

                Inventory theInventory = new Inventory();
                theInventory.setItemName(sArray[0]);
                theInventory.setSku(sArray[1]);
                theInventory.setVendorPrice(
                    (new Float(sArray[2])).floatValue());
                theInventory.setVendorInventory(
                    (new Integer(sArray[3])).intValue());
                theInventory.setCategory(sArray[4]);
                theInventory.setVendor(sArray[5]);

                // Put it in the store. Note that this causes our secondary key
                // to be automatically updated for us.
                try {
                      da.inventoryBySku.put(txn, theInventory);
                } catch (DatabaseException dbe) {
                    System.out.println("Error putting entry " +
                        sku.getBytes("UTF-8"));
                    txn.abort();
                    throw dbe;
                }
            }
            // Commit the transaction. The data is now safely written to the
            // store.
            txn.commit();
        }

The remainder of this example simple parses the command line and loads data from a flat-text file. There is nothing here that is of specific interest to the DPL, but we show this part of the example anyway in the interest of completeness.

        private static void parseArgs(String args[]) {
            for(int i = 0; i < args.length; ++i) {
                if (args[i].startsWith("-")) {
                    switch(args[i].charAt(1)) {
                      case 'h':
                        myDbEnvPath = new File(args[++i]);
                        break;
                      case 'i':
                        inventoryFile = new File(args[++i]);
                        break;
                      case 'v':
                        vendorsFile = new File(args[++i]);
                        break;
                      default:
                        usage();
                    }
                }
            }
        }

        private List loadFile(File theFile, int numFields) {
            List<String[]> records = new ArrayList<String[]>();
            try {
                String theLine = null;
                FileInputStream fis = new FileInputStream(theFile);
                BufferedReader br =
                    new BufferedReader(new InputStreamReader(fis));
                while((theLine=br.readLine()) != null) {
                    String[] theLineArray = theLine.split("#");
                    if (theLineArray.length != numFields) {
                        System.out.println("Malformed line found in " +
                            theFile.getPath());
                        System.out.println("Line was: '" + theLine);
                        System.out.println("length found was: " +
                            theLineArray.length);
                        System.exit(-1);
                    }
                    records.add(theLineArray);
                }
                // Close the input stream handle
                fis.close();
            } catch (FileNotFoundException e) {
                System.err.println(theFile.getPath() + " does not exist.");
                e.printStackTrace();
                usage();
            } catch (IOException e)  {
                System.err.println("IO Exception: " + e.toString());
                e.printStackTrace();
                System.exit(-1);
            }
            return records;
        }

        protected ExampleDatabasePut() {}
    }

This completes our example application. Be aware that this example and supporting classes can be found in your JE distribution in the following location:

JE_HOME/examples/persist/gettingStarted

where `JE_HOME` is the location where you placed your JE distribution.

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