Android Internals and the Dalvik VM!

Adam Champion, Andy Pyles, Boxuan Gu!

Derived in part from presentations by Patrick Brady, Dan Bornstein, and Dan Morrill from Google (http://source.android.com/documentation)! Introduction!

• Android internals (e.g., Linux kernel, libraries, etc.)! • Dalvik Virtual Machine! • Android application framework! How everything fits together! Android Internals - Linux Kernel!

• Android runs on the Linux 2.6.27 kernel -- but Android is not Linux!! o No GNU libc (discussed on next slide)! o No windowing system ! • Android extends Linux with custom IPC framework (Binder) and aggressive power management! o Binder is Google's lightweight IPC driver that uses shared memory for high performance, synch. process calls to maintain single-process programming model ! o (Linux) Power management: shuts down CPU/screen to save battery power unless app requests that either CPU or both CPU and display stay on (wake locks) ! Android Internals - Native Libraries!

• Android uses these C/C++ libraries (among others):! o Bionic, Google's libc implementation optimized for embedded software platforms (BSD license)! o WebKit open-source browser engine (powers Safari)! o SQLite relational data store (stores all persistent data)! o Media framework (supports many A/V codecs)! o SurfaceFlinger graphics compositor to framebuffer! o AudioFlinger audio "compositor" to earpiece/headset/...! • All these libraries sit atop the Hardware Abstraction Layer (HAL), where handset manufacturers provide native code for graphics, audio, GPS, etc.! o The HAL runs in user space, not kernel space! o Copes with non-standard interface specs, IP issues ! Android Internals - Runtime !

• Android's runtime runs atop these libraries. Comprises Dalvik VM and core libraries exposed via Java (JNI)! o Dalvik VM designed for ! ! App portability on different hardware implementations! ! CPU/memory efficiency ! o All Android apps run atop Dalvik ! o Core libraries provide standard Java API for app developers, e.g.,! ! Data structure! ! File & network access! ! Graphics! o They "plug in" to Dalvik ! Application Framework Terminology (1)!

• Android PacKage (APK) is a collection of components! o The APK file is a Zip file that contains source code files, resources (e.g., images, GUI layouts), etc.! o Components share databases, file space, prefs, etc.! o Each APK has an associated Linux process with a unique UID! o There's usually 1 thread per process! • Terminology: Apps are characterized by Activities, Tasks, and Processes! o An Activity is a discrete chunk of functionality (concrete class that encapsulates some operation)! o A Task contains one or more Activities (a.k.a. an app)! o A Process is a Linux process! Application Framework Terminology (2)!

• A process for a UID is launched "as needed"! o Binding to a Service or ContentProvider! o Launching an Activity! o Firing an IntentReceiver ! • Process runs until Android kills it to save memory! • Each Activity has a managed lifecycle! o Started when requested! o Can lose focus or visibility if, e.g., a dialog box pops up, user launches another app! o Android nondeterministically kills Activity to free up memory! Activity life cycle! Android Application Security!

• As previously mentioned, each application has its own UID. ! • By default, an application has zero permissions. Must explicitly declare permissions in AndroidManifest.xml file! • For instance, an app that monitors incoming SMS messages must put the following in that file:! ! # ! • Each app must also be signed with a certificate for which the developer has the private key.! Dalvik VM Overview!

• Motivation: VM must run with low memory, limited CPU power, no swap space, and while powered by a battery! • While the Android SDK uses Java code, Java bytecode (.class files) and JAR archives are slightly bloated! • Dalvik uses Dalvik EXecutable (.dex) custom bytecode to compress bytecode constants & data efficiently! • Strategy: "Zygote" process launches at boot, maximizes shared memory! o Minimizes use of "dirty" heap memory by creating heap at boot, using shared memory as much as possible! o Zygote preloads common Android classes, libraries! o It listens for new processes and fork()s on demand ! • Garbage Collection - mark bits separated from object data! Overview of Dalvik VM Compilation and Interpretation! Dex overview! public static long sumArray(int[] arr) # { long sum = 0; # for (int i : arr) # { sum += i; }#

return sum; }# # .class bytecode output!

! • 25 bytes, 45 reads, 16 writes! ! 0000: lconst_0# 0001: lstore_1 0002: aload_0 0003: astore_3 0004: aload_3 0005: arraylength# 0006: istore 04 0008: iconst_0# 0009: istore 05 000b: iload 05 // rl ws 000d: iload 04 // rl ws 000f: if_icmpge 0024 // rs rs 0012: aload_3 // rl ws 0013: iload 05 // rl ws 0015: iaload // rs rs ws 0016: istore 06 // rs wl 0018: lload_1 // rl rl ws ws 0019: iload 06 // rl ws# 001b: i2l // rs ws ws 001c: ladd // rs rs rs rs ws ws 001d: lstore_1 // rs rs wl wl // ws = write stack 001e: iinc 05, #+01 // rl wl 0021: goto 000b // rl = read local // wl = write local 0024: lload_1 0025: lreturn# .dex bytecode output! ! • 18 bytes, 19 reads, 6 writes! ! ! 0000: const-wide/16 v0, #long 0 0002: array-length v2, v8 0003: const/4 v3, #int 0 0004: move v7, v3 # 0005: move-wide v3, v0 0006: move v0, v7 0007: if-ge v0, v2, 0010 0009: aget v1, v8, v0 000b: int-to-long v5, v1 000c: add-long/2addr v3, v5 000d: add-int/lit8 v0, v0, #int 1 000f: goto 0007 0010: return-wide v3# # Questions & Comments?!

! ! Thank you!!