Table of Contents
JE records contain two parts — a key and some data. Both the key
and its corresponding data are
encapsulated in
DatabaseEntry class objects.
Therefore, to access a JE record, you need two such
objects, one for the key and
one for the data.
DatabaseEntry can hold any kind of data from simple
Java primitive types to complex Java objects so long as that data can be
represented as a Java byte array. Note that due to
performance considerations, you should not use Java serialization to convert
a Java object to a byte array. Instead, use the Bind APIs
to perform this conversion (see
Using the BIND APIs for more
information).
This chapter describes how you can convert both Java primitives and Java
class objects into and out of byte arrays. It also
introduces storing and retrieving key/value pairs from a database. In
addition, this chapter describes how you can use comparators to influence
how JE sorts its database records.
Each database record is comprised of two
DatabaseEntry objects
— one for the key and another for the data.
The key and data information are passed to-
and returned from JE using
DatabaseEntry objects as byte
arrays. Using DatabaseEntrys allows JE to
change the underlying byte array as well as return multiple values (that
is, key and data). Therefore, using DatabaseEntry instances
is mostly an exercise in efficiently moving your keys and your data in
and out of byte arrays.
For example, to store a database record where both the key and the
data are Java String objects, you instantiate a
pair of DatabaseEntry objects:
package je.gettingStarted;
import com.sleepycat.je.DatabaseEntry;
...
String aKey = "key";
String aData = "data";
try {
DatabaseEntry theKey = new DatabaseEntry(aKey.getBytes("UTF-8"));
DatabaseEntry theData = new DatabaseEntry(aData.getBytes("UTF-8"));
} catch (Exception e) {
// Exception handling goes here
}
// Storing the record is described later in this chapter
Notice that we specify UTF-8 when we retrieve the
byte array from our String
object. Without parameters, String.getBytes() uses the
Java system's default encoding. You should never use a system's default
encoding when storing data in a database because the encoding can change.
When the record is retrieved from the database, the method that you
use to perform this operation populates two DatabaseEntry
instances for you, one for the key and another for the data. Assuming Java
String objects, you retrieve your data from the
DatabaseEntry as follows:
package je.gettingStarted; import com.sleepycat.je.DatabaseEntry; ... // theKey and theData are DatabaseEntry objects. Database // retrieval is described later in this chapter. For now, // we assume some database get method has populated these // objects for us. // Use DatabaseEntry.getData() to retrieve the encapsulated Java // byte array. byte[] myKey = theKey.getData(); byte[] myData = theData.getData(); String key = new String(myKey, "UTF-8"); String data = new String(myData, "UTF-8");
There are a large number of mechanisms that you can use to move data in
and out of byte arrays. To help you with this
activity, JE provides the bind APIs. These APIs allow you to
efficiently store both primitive data types and complex objects in
byte arrays.
The next section describes basic database put and get operations. A basic understanding of database access is useful when describing database storage of more complex data such as is supported by the bind APIs. Basic bind API usage is then described in Using the BIND APIs.