Dalvik (software): Difference between revisions

Dalvik is a process virtual machine (VM) in Google's Android operating system that executes applications written for Android. This makes Dalvik an integral part of the Android software stack (in Android versions 4.4 "KitKat" and earlier) that is typically used on mobile devices such as mobile phones and tablet computers, as well as more recently on devices such as smart TVs and wearables.

Programs are commonly written in Java and compiled to bytecode for the Java virtual machine, which is then translated to Dalvik bytecode and stored in .dex (Dalvik EXecutable) and .odex (Optimized Dalvik EXecutable) files; related terms odex and de-odex are associated with respective bytecode conversions. The compact Dalvik Executable format is designed for systems that are constrained in terms of memory and processor speed.

Dalvik is open-source software. It was originally written by Dan Bornstein, who named it after the fishing village of Dalvík in Eyjafjörður, Iceland.^[1][2]

An alternative runtime environment called Android Runtime (ART) was included in Android 4.4 "KitKat" as a technology preview.^[3][4] ART replaces Dalvik entirely in Android 5.0 "Lollipop".^[5]

Unlike Java VMs, which are stack machines, the Dalvik VM uses a register-based architecture that requires fewer, typically more complex virtual machine instructions. Dalvik programs are written in Java using the Android application programming interface (API), compiled to Java bytecode, and converted to Dalvik instructions as necessary.

A tool called dx is used to convert Java .class files into the .dex format. Multiple classes are included in a single .dex file. Duplicate strings and other constants used in multiple class files are included only once in the .dex output to conserve space. Java bytecode is also converted into an alternative instruction set used by the Dalvik VM. An uncompressed .dex file is typically a few percent smaller in size than a compressed Java archive (JAR) derived from the same .class files.^[6]

The Dalvik executables may be modified again when installed onto a mobile device. In order to gain further optimizations, byte order may be swapped in certain data, simple data structures and function libraries may be linked inline, and empty class objects may be short-circuited, for example.

Being optimized for low memory requirements, Dalvik has some specific characteristics that differentiate it from other standard VMs:^[7]

• The VM was slimmed down to use less space.
• The constant pool has been modified to use only 32-bit indices to simplify the interpreter.
• Standard Java bytecode executes 8-bit stack instructions. Local variables must be copied to or from the operand stack by separate instructions. Dalvik instead uses its own 16-bit instruction set that works directly on local variables. The local variable is commonly picked by a 4-bit 'virtual register' field. This lowers Dalvik's instruction count and raises its interpreter speed.

According to Google, the design of Dalvik permits a device to run multiple instances of the VM efficiently.^[8]

Android 2.2 "Froyo" brought trace-based just-in-time (JIT) compilation into Dalvik, optimizing the execution of applications by continually profiling applications each time they run and dynamically compiling frequently executed short segments of their bytecode into native machine code. While Dalvik interpretes the rest of application's bytecode, native execution of those short bytecode segments, called "traces", provides significant performance improvements.^[9][10][11]

The relative merits of stack machines versus register-based approaches are a subject of ongoing debate.^[12]

Generally, stack-based machines must use instructions to load data on the stack and manipulate that data, and, thus, require more instructions than register machines to implement the same high level code, but the instructions in a register machine must encode the source and destination registers and, therefore, tend to be larger. This difference is primarily of importance to VM interpreters for which opcode dispatch tends to be expensive along with other factors similarly relevant to just-in-time compilation.

However, tests performed on ARM devices in 2010 by Oracle (owner of the Java technology) with standard non-graphical Java benchmarks on Java SE embedded seemed to show it to be two to three times faster than Android 2.2, which is the initial release to include a just-in-time (JIT) compiler.^{[13][needs update]}

In 2012, academic benchmarks confirmed the factor of 3 between HotSpot and Dalvik on the same Android board, also noting that Dalvik code was not smaller than Hotspot.^[14]

Furthermore, benchmarks performed on Android device still show (as of March 2014) up to a factor 100 between native applications and a Dalvik application on the same Android device.^{[15][original research?][improper synthesis?]} Upon running benchmarks using the early interpreter of 2009, both JNI and native code showed an order of magnitude speed up.^[16]

Dalvik is published under the terms of the Apache License 2.0.^[17] Google says that Dalvik is a clean-room implementation rather than a development on top of a standard Java runtime, which would mean it does not inherit copyright-based license restrictions from either the standard-edition or open-source-edition Java runtimes.^[18] Oracle and some reviewers dispute this.^[19]

On August 12, 2010, Oracle, which acquired Sun Microsystems in April 2009 and therefore owns the rights to Java, sued Google over claimed infringement of copyrights and patents. Oracle alleged that Google, in developing Android, knowingly, directly and repeatedly infringed Oracle's Java-related intellectual property.^[20][21][22] In May 2012, the jury in this case found that Google did not infringe on Oracle's patents, and the trial judge ruled that the structure of the Java APIs used by Google was not copyrightable.^[23][24] The parties agreed to zero dollars in statutory damages for 9 lines of copied code.^[25][26]

On May 9, 2014, the Federal Circuit partially reversed the district court ruling, ruling in Oracle's favor on the copyrightability issue, and remanding the issue of fair use back to the district court.^[27][28]

• Android software development
• Application virtualization
• Comparison of application virtual machines
• Comparison of Java and Android API
• JEB Decompiler Dalvik (DEX and APK) Decompiler
• Dalvik Turbo virtual machine, a proprietary alternative Dalvik implementation

• Dalvik Instruction Set – official Android documentation
• Dex Executable Format – official Android documentation
• Pamela Jones (August 16, 2010). "Oracle America's complaint against Google, as text". Groklaw.
• A JIT Compiler for Android's Dalvik VM on YouTube, Google I/O 2010, by Ben Cheng and Bill Buzbee
• Dalvik VM Internals, Google I/O 2008, by Dan Bornstein
• Oracle v Google Copyright Claims Decompiled