Dalvik VM in Android

Dalvik Virtual Machine (DVM) was a crucial component of the Android operating system architecture prior to Android 5.0 (Lollipop). It played a central role in running Android apps and handling their execution. However, with the introduction of Android 5.0, the Android Runtime (ART) replaced the Dalvik Virtual Machine as the default runtime environment for Android apps.

Here are the key points about the Dalvik Virtual Machine in Android:

1. Purpose: The Dalvik Virtual Machine was specifically designed for Android to execute Android application code. It was optimized for resource-constrained mobile devices, such as smartphones and tablets.
2. Dex Files: Android apps are written in Java, but the Dalvik VM did not execute standard Java bytecode. Instead, Android apps were compiled into a special format called DEX (Dalvik Executable) files. DEX files are more compact and optimized for mobile devices compared to standard Java class files.
3. Just-In-Time (JIT) Compilation: The Dalvik VM used a Just-In-Time (JIT) compilation approach, which means that DEX bytecode was translated into native machine code at runtime. This allowed for faster execution of apps but incurred a performance penalty during the initial startup as code had to be compiled.
4. Process Isolation: Each Android app ran in its own instance of the Dalvik VM, providing process isolation. This ensured that one app couldn’t directly interfere with the memory or execution of another app.
5. Memory Management: The Dalvik VM used a garbage collector to manage memory and automatically reclaim memory occupied by objects that were no longer needed.
6. Optimizations: Dalvik included several optimizations to improve app performance on mobile devices, such as efficient memory management, streamlined garbage collection, and support for low-memory devices.
7. Security: Dalvik contributed to the security of the Android platform by isolating apps and their execution environments from each other.
8. Obsolete: With the release of Android 5.0 (Lollipop), Google introduced the Android Runtime (ART) as the default runtime for Android. ART brought ahead-of-time (AOT) compilation, which compiled apps into native code at installation rather than at runtime. This change improved app performance and battery efficiency.
9. Transition to ART: Starting with Android 5.0, ART became the default runtime, and Dalvik was phased out. Most Android devices running Android 5.0 or later use ART as the runtime environment.
10. Legacy Support: Some older Android devices may still use Dalvik VM, but its use is increasingly rare as newer versions of Android are adopted.

In summary, the Dalvik Virtual Machine was a fundamental component of Android’s architecture that executed Android app code in a unique DEX bytecode format. However, it was replaced by the Android Runtime (ART) in newer Android versions to improve app performance, battery life, and overall efficiency.