What is difference between JVM and DVM? Explain in detail.
Java Virtual Machine (JVM) and Dalvik Virtual Machine (DVM) are both virtual machines used to execute Java and Android applications, respectively. However, there are key differences between the two, as they are designed to cater to different environments. Here’s a detailed comparison between JVM and DVM:
| Feature | JVM | DVM |
|---|---|---|
| Platform | JVM is designed for executing Java applications on various platforms. | DVM is specifically designed for the Android operating system to run Android applications. |
| Bytecode Format | JVM executes Java bytecode, which is generated by compiling Java source code. | DVM executes Dalvik bytecode (DEX), a format optimized for Android applications. DEX files are generated from Java bytecode during the Android app build process. |
| Compilation Model | JVM uses just-in-time (JIT) compilation. Bytecode is translated into native machine code at runtime. | DVM used just-in-time (JIT) compilation in earlier versions. However, later versions introduced the Android Runtime (ART), which uses ahead-of-time (AOT) compilation during app installation. |
| Architecture | JVM uses a stack-based architecture. Operations are performed using a stack data structure. | DVM uses a register-based architecture. Operations are performed directly on registers, which can be more efficient in certain scenarios. |
| Optimizations | JVM performs various runtime optimizations, including just-in-time compilation, garbage collection, and hotspot analysis. | DVM performs runtime optimizations, including trace-based just-in-time compilation, garbage collection, and other optimizations to improve the performance of Android applications. |
| Garbage Collection | JVM includes a garbage collector for managing memory and reclaiming memory occupied by objects no longer in use. | DVM also includes a garbage collector to manage memory and prevent memory leaks in Android applications. |
| Multithreading Support | JVM supports multithreading, allowing multiple threads to execute concurrently. | DVM supports multithreading, enabling Android applications to execute multiple threads concurrently. |
| Security Model | JVM enforces security measures, including class loaders and the Security Manager, to ensure a secure execution environment. | DVM contributes to the overall security of the Android platform by enforcing application sandboxing and preventing unauthorized access between apps. |
| Compatibility | JVM is designed to be platform-independent, allowing Java applications to run on various devices with a JVM installed. | DVM is specifically designed for Android, providing compatibility with Java bytecode and allowing Android apps to run on a range of Android devices. |
| Tooling | JVM has a rich ecosystem of development tools and debugging utilities available for Java developers. | DVM has development tools and debugging utilities specifically tailored for Android app development. Android Studio and adb (Android Debug Bridge) are commonly used tools. |
| 64-Bit Support | JVM supports both 32-bit and 64-bit architectures. | DVM, being replaced by ART, may have limitations in 64-bit support in older Android versions. ART, the successor to DVM, fully supports 64-bit architectures. |
It’s important to note that with the introduction of the Android Runtime (ART) in Android 5.0 (Lollipop), DVM has been largely deprecated. ART brought significant improvements in performance, memory efficiency, and overall system responsiveness to the Android platform. Developers are encouraged to use ART for Android app development.