15.8.1 CDS Overview Explained
Class Data Sharing (CDS) is a feature introduced in Java SE 11 that aims to improve the startup time and reduce the memory footprint of Java applications. CDS allows multiple JVMs to share a common set of classes in memory, which can significantly enhance performance and resource utilization.
Key Concepts
1. Class Data Sharing (CDS)
CDS is a mechanism that enables the sharing of class metadata between multiple JVM instances. By storing the metadata of frequently used classes in a shared archive, CDS reduces the time and resources required to load these classes, leading to faster application startup and lower memory usage.
Example
java -Xshare:dump java -Xshare:on -jar MyApplication.jar
2. Shared Archive
A shared archive is a file that contains the metadata of classes that can be shared between JVMs. The JVM creates this archive during the first run and uses it in subsequent runs to load classes more efficiently.
Example
java -Xshare:dump java -Xshare:on -jar MyApplication.jar
3. Startup Time Improvement
By using CDS, the JVM can load classes from the shared archive instead of parsing and loading them individually. This reduces the time required to start the application, making it more responsive.
Example
Before CDS: Application startup time = 5 seconds After CDS: Application startup time = 2 seconds
4. Memory Footprint Reduction
Since the shared archive contains pre-loaded class metadata, multiple JVMs can share this data, reducing the overall memory footprint. This is particularly beneficial in environments with multiple JVM instances running concurrently.
Example
Before CDS: Memory usage per JVM = 500 MB After CDS: Memory usage per JVM = 400 MB
Examples and Analogies
Think of CDS as a library of frequently used books. Instead of each student (JVM instance) bringing their own copy of the books, they share a common library (shared archive). This reduces the time it takes to get the books (load classes) and the space required to store them (memory footprint).
For instance, if you are running multiple instances of a web application, using CDS can help reduce the startup time and memory usage of each instance. This makes the application more efficient and responsive, especially in high-concurrency environments.
Example
java -Xshare:dump java -Xshare:on -jar WebApplication.jar
By mastering the concepts of Class Data Sharing, you can optimize the performance and resource utilization of your Java applications, making them more efficient and scalable.