Test App: Add Java code for transaction retries

Add example code for transaction retry logic. Compilation and
execution instructions are in a comment in the main code file.
Fixes cockroachdb#757.

_includes/app/txn-sample.java

@@ -0,0 +1,88 @@
+import java.sql.*;
+
+/*
+
+  TODO(jseldess): This file needs to be renamed to TxnSample.java to
+  fit with Java naming conventions (see BasicSample.java). I've held
+  off on that as that needs renamings on the rest of the doc page.
+
+  You can compile and run this example with a command like:
+  javac TxnSample.java && java -cp .:~/path/to/postgresql-9.4.1208.jar TxnSample
+  You can download the postgres JDBC driver jar from https://jdbc.postgresql.org.
+*/
+
+class InsufficientBalanceException extends Exception {}
+
+// A simple interface that provides a retryable lambda expression.
+interface RetryableTransaction {
+    public void run(Connection conn) throws Exception;
+}
+
+public class TxnSample {
+    public static RetryableTransaction transferFunds(int from, int to, int amount) {
+        return new RetryableTransaction() {
+            public void run(Connection conn) throws Exception {
+                ResultSet res = conn.createStatement().executeQuery("SELECT balance FROM accounts WHERE id = " + from);
+                res.next();
+                int balance = res.getInt("balance");
+                if(balance < from) {
+                    throw new InsufficientBalanceException();
+                }
+                conn.createStatement().executeUpdate("UPDATE accounts SET balance = balance - " + amount + " where id = " + from);
+                conn.createStatement().executeUpdate("UPDATE accounts SET balance = balance + " + amount + " where id = " + to);
+            }
+        };
+    }
+
+    public static void retryTransaction(Connection conn, RetryableTransaction tx) throws Exception {
+        Savepoint sp = conn.setSavepoint("cockroach_restart");
+        while(true) {
+            try {
+                // Attempt the transaction.
+                tx.run(conn);
+
+                // If we reach this point, commit the transaction,
+                // which implicitly releases the savepoint.
+                conn.commit();
+                break;
+            } catch(SQLException e) {
+                // Check if the error code indicates a SERIALIZATION_FAILURE.
+                if(e.getErrorCode() == 40001) {
+                    // Signal the database that we will attempt a retry.
+                    conn.rollback(sp);
+                }
+                // This is a not a serialization failure, pass it up the chain.
+                throw e;
+            }
+        }
+    }
+
+    public static void main(String[] args) throws ClassNotFoundException, SQLException {
+        // Load the postgres JDBC driver.
+        Class.forName("org.postgresql.Driver");
+
+        // Connect to the "bank" database.
+        Connection db = DriverManager.getConnection("jdbc:postgresql://127.0.0.1:26257/bank?sslmode=disable", "maxroach", "");
+            try {
+                // We need to turn off autocommit mode to allow for
+                // multi-statement transactions.
+                db.setAutoCommit(false);
+
+                RetryableTransaction transfer = transferFunds(1, 2, 100);
+                retryTransaction(db, transfer);
+                ResultSet res = db.createStatement().executeQuery("SELECT id, balance FROM accounts");
+                while (res.next()) {
+                    System.out.printf("\taccount %s: %s\n", res.getInt("id"), res.getInt("balance"));
+                }
+            } catch(InsufficientBalanceException ibe) {
+                System.out.println("Insufficient balance");
+            } catch(SQLException sqle) {
+                System.out.println("SQLException encountered:" + sqle);
+            } catch(Exception e) {
+                System.out.println("Unknown exception encountered:" + e);
+            } finally {
+                // Close the database connection.
+                db.close();
+            }
+    }
+}