Table of Contents
You store records in JE as key/data pairs. Both the key and the value are encapsulated in DatabaseEntry class objects.
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.
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 is stored in DatabaseEntry objects as byte arrays. 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:
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:
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); String data = new String(myData);
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.