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Android: javac vs Dalvik

由 坚强是说给别人听的谎言 提交于 2019-11-29 19:39:01
user370305
I'm trying to understand what types of Java code will not run when "fed" into the Dalvik JVM.

Dalvik JVM differs from other JVMs in following aspects:

  • It uses special DEX format for storing applications binaries vs. JAR and Pack200 formats used by standard Java virtual machines. Google claims that DEX results in smaller binaries than JAR. I think they could use Pack200 with the same success, but they decided to go their own way in this aspect

  • Dalvik JVM was optimized for running multiple JVM processes simultaneously

  • Dalvik JVM uses register-based architecture vs. stack based architecture of other JVMs with intent to speed up execution and to reduce binary sizes

  • It uses its own instructions set (not a standard JVM bytecode)

  • One can run (if needed) several independent Android applications within a single JVM process

  • Application execution can span across several Dalvik JVM processes “naturally”. To support this it adds:

    • Special object serialization mechanism based on Parcel and Parcelable classes. Functionally it serves the same purpose as standard Java Serializable, but results in smaller data footprint and is potentially more lenient towards differences in versions of classes

    • Special Android way to execute inter process calls (IPC) based on Android Interface Definition Language (AIDL)

  • Until Android 2.2 Dalvik JVM did not support JIT compilation which adversely impacted Android application performance. Adding it in 2.2 improves markedly execution speed for often used applications

If anything I have said so far is incorrect, please begin by correcting me!

Ummm, well...

  • The Dalvik VM has technical advantages over the Java VM for mobile environments, most notably aggressive use of copy-on-write memory sharing, so the entire VM and standard class library is shared among all Android SDK app processes, reducing the net per-process memory footprint. See user370305's answer (posted while I was wrapping this up) for more.

  • The bytecode from javac is cross-compiled into Dalvik bytecode as part of the Android application build process. The Java VM cannot execute Dalvik bytecode any more than it can execute the output of /dev/random; similarly, the Dalvik VM cannot execute Java bytecode.

Here is a blog post of mine from around two years ago that goes into additional points.

If javac is used to compile all sources into bytecode, then why is it that Dalvik can't run some types of Java code?

Because the javac bytecode output is cross-compiled. The cross-compiler (dx) handles a very specific flavor of javac output, meaning that while it works with the classic javac (what you would have gotten from java.sun.com) and OpenJDK for Java 1.5 and 1.6, it will not work with alternative compilers (e.g., GCJ) and, at minimum, will not work with any new bytecodes from Java 7.

no one was able to explain what it is that's inherent to Dalvik that makes it impossible to run Java code from projects like Google Guice or Apache Camel

Personally, I have never used Google Guice, though Roboguice works on Android. I had never heard of Apache Camel prior to your question and am rather confused to find that it is not a Java port of Perl. :-)

Any tools that do runtime JVM bytecode generation will not work on Android, simply because the cross-compiler is only available at compile-time, not run-time. Also, I am unfamiliar with the techniques used by runtime JVM bytecode-generating tools and how they get the JVM to execute that bytecode, and therefore I do not know if equivalent hooks exist in Android to have Dalvik run arbitrary chunks of Dalvik bytecode.

However, since you declined to specify exactly what "Java code from projects like Google Guice or Apache Camel" you are having problems with, and since I am not intimately familiar with those projects, it is difficult to comment further.

This picture from Android official document illustrate the build process of Android APK, it will help to understand the difference between java bytecode and dalvik executable.

Here I give an example to demonstrate some of the differences.

Hello.java

import java.io.*;
public class Hello {
    public static void main(String[] args) {
        System.out.println("hello world!!!!");
    }
}

use javac to compile Hello.java to java bytecode Hello.class

$ javac Hello.java

Then use dx tool from android sdk convert java bytecode Hello.class to Hello.dex

$ $ANDROID_SDK_ROOT/build-tools/21.1.2/dx --dex --output=Hello.dex Hello.class

After that, use adb to put Hello.class and Hello.dex to Android device or emulator.

$ adb push Hello.class /data/local/tmp/
$ adb push Hello.dex /data/local/tmp/

use adb shell to enter the shell environment of Android device. Then use the command /system/bin/dalvikvm to execute the simple java program we just created Hello.class and Hello.dex

$ dalvikvm -Djava.class.path=./Hello.class Hello
java.lang.NoClassDefFoundError: Hello
    at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.ClassNotFoundException: Didn't find class "Hello" on path: ./Hello.class
    at dalvik.system.BaseDexClassLoader.findClass(BaseDexClassLoader.java:65)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:501)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:461)
    ... 1 mor
$ dalvikvm -Djava.class.path=./Hello.dex Hello   
hello world!!!!

In the example above, when we use the java bytecode Hello.class, dalvikvm complaint error, if we changed the class to dalvik executable Hello.dex, it would run properly.

标签
java
android
javac
dalvik
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