am f2960b8d: Refresh the layoutlib_create doc.
Merge commit 'f2960b8d66d7a20f590716f5e51160e77d0d1b24' into gingerbread-plus-aosp * commit 'f2960b8d66d7a20f590716f5e51160e77d0d1b24': Refresh the layoutlib_create doc.
This commit is contained in:
Raphael Moll
Android Git Automerger
@ -4,68 +4,196 @@
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- Description -
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---------------
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makeLayoutLib generates a library used by the Eclipse graphical layout editor
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Layoutlib_create generates a JAR library used by the Eclipse graphical layout editor
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to perform layout.
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- Usage -
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---------
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./makeLayoutLib path/to/android.jar destination.jar
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./layoutlib_create path/to/android.jar destination.jar
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- Design Overview -
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-------------------
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Layoutlib_create uses the "android.jar" containing all the Java code used by Android
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as generated by the Android build, right before the classes are converted to a DEX format.
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The Android JAR can't be used directly in Eclipse:
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- it contains references to native code (which we want to avoid in Eclipse),
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- some classes need to be overridden, for example all the drawing code that is
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replaced by Java 2D calls in Eclipse.
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- some of the classes that need to be changed are final and/or we need access
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to their private internal state.
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Consequently this tool:
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- parses the input JAR,
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- modifies some of the classes directly using some bytecode manipulation,
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- filters some packages and removes some that we don't want to end in the output JAR,
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- injects some new classes,
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- and generates a modified JAR file that is suitable for the Android plugin
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for Eclipse to perform rendering.
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The ASM library is used to do the bytecode modification using its visitor pattern API.
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The layoutlib_create is *NOT* generic. There is no configuration file. Instead all the
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configuration is done in the main() method and the CreateInfo structure is expected to
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change with the Android platform as new classes are added, changed or removed.
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The resulting JAR is used by layoutlib_bridge (a.k.a. "the bridge"), also part of the
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platform, that provides all the necessary missing implementation for rendering graphics
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in Eclipse.
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- Implementation Notes -
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------------------------
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The goal of makeLayoutLib is to list all the classes from the input jar and create a new
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jar that only keeps certain classes and create stubs for all their dependencies.
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First the input jar is parsed to find all the classes defined.
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In the Main(), the following list of classes are hardcoded (TODO config file later):
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- keep all classes that derive from android.view.View.
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- keep all classes in the android.view and android.widget packages (sub-packages excluded).
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- keep specific classes such as android.policy.PhoneLayoutInflater.
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For each class to keep, their dependencies are examined using BCEL.
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A dependency is defined as a class needed to instantiate the given class that should be kept,
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directly or indirectly. So a dependency is a class that is used by the input class, that is
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defined in the input jar and that is not part of the current JRE.
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Dependencies are computed recursively.
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Once all dependencies are found, the final jar can be created.
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There are three kind of classes to write:
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- classes that are to be kept as-is. They are just dumped in the new jar unchanged.
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- classes that are to be kept yet contain native methods or fields.
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- classes that are just dependencies. We don't want to expose their implementation in the final
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jar.
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The implementation of native methods and all methods of mock classes is replaced by a stub
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that throws UnsupportedOperationException.
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Incidentally, the access level of native and mock classes needs to be changed in order for
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native methods to be later overridden. Methods that are "final private native" must become
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non-final, non-native and at most protected. Package-default access is changed to public.
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Classes that are final are made non-final. Abstract methods are left untouched.
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The tool works in two phases:
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- first analyze the input jar (AsmAnalyzer class)
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- then generate the output jar (AsmGenerator class),
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- Analyzer
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----------
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----
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20080617 Replace Class
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The goal of the analyzer is to create a graph of all the classes from the input JAR
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with their dependencies and then only keep the ones we want.
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Some classes are basically wrappers over native objects.
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Subclassing doesn't work as most methods are either static or we don't
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control object creation. In this scenario the idea is to be able to
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replace classes in the final jar.
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To do that, the analyzer is created with a list of base classes to keep -- everything
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that derives from these is kept. Currently the one such class is android.view.View:
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since we want to render layouts, anything that is sort of the view needs to be kept.
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Example: android.graphics.Paint would get renamed to OriginalPaint
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in the generated jar. Then in the bridge we'll introduce a replacement
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Paint class that derives from OriginalPaint.
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The analyzer is also given a list of class names to keep in the output.
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This is done using shell-like glob patterns that filter on the fully-qualified
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class names, for example "android.*.R**" ("*" does not matches dots whilst "**" does,
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and "." and "$" are interpreted as-is).
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In practice we almost but not quite request the inclusion of full packages.
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With this information, the analyzer parses the input zip to find all the classes.
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All classes deriving from the requested bases classes are kept.
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All classes which name matched the glob pattern are kept.
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The analysis then finds all the dependencies of the classes that are to be kept
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using an ASM visitor on the class, the field types, the method types and annotations types.
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Classes that belong to the current JRE are excluded.
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The output of the analyzer is a set of ASM ClassReader instances which are then
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fed to the generator.
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- Generator
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-----------
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The generator is constructed from a CreateInfo struct that acts as a config file
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and lists:
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- the classes to inject in the output JAR -- these classes are directly implemented
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in layoutlib_create and will be used to interface with the renderer in Eclipse.
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- specific methods to override (see method stubs details below).
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- specific methods to remove based on their return type.
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- specific classes to rename.
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Each of these are specific strategies we use to be able to modify the Android code
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to fit within the Eclipse renderer. These strategies are explained beow.
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The core method of the generator is transform(): it takes an input ASM ClassReader
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and modifies it to produce a byte array suitable for the final JAR file.
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The first step of the transformation is changing the name of the class in case
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we requested the class to be renamed. This uses the RenameClassAdapter to also rename
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all inner classes and references in methods and types. Note that other classes are
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not transformed and keep referencing the original name.
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The TransformClassAdapter is then used to process the potentially renamed class.
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All protected or private classes are market as public.
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All classes are made non-final.
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Interfaces are left as-is.
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If a method has a return type that must be erased, the whole method is skipped.
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Methods are also changed from protected/private to public.
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The code of the methods is then kept as-is, except for native methods which are
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replaced by a stub. Methods that are to be overridden are also replaced by a stub.
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Finally fields are also visited and changed from protected/private to public.
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- Method stubs
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--------------
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As indicated above, all native and overridden methods are replaced by a stub.
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We don't have the code to replace with in layoutlib_create.
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Instead the StubMethodAdapter replaces the code of the method by a call to
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OverrideMethod.invokeX(). When using the final JAR, the bridge can register
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listeners from these overridden method calls based on the method signatures.
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The listeners are currently pretty basic: we only pass the signature of the
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method being called, its caller object and a flag indicating whether the
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method was native. We do not currently provide the parameters. The listener
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can however specify the return value of the overridden method.
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An extension being worked on is to actually replace these listeners by
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direct calls to a delegate class, complete with parameters.
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- Strategies
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------------
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We currently have 4 strategies to deal with overriding the rendering code
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and make it run in Eclipse. Most of these strategies are implemented hand-in-hand
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by the bridge (which runs in Eclipse) and the generator.
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1- Class Injection
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This is the easiest: we currently inject 4 classes, namely:
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- OverrideMethod and its associated MethodListener and MethodAdapter are used
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to intercept calls to some specific methods that are stubbed out and change
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their return value.
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- CreateInfo class, which configured the generator. Not used yet, but could
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in theory help us track what the generator changed.
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2- Overriding methods
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As explained earlier, the creator doesn't have any replacement code for
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methods to override. Instead it removes the original code and replaces it
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by a call to a specific OveriddeMethod.invokeX(). The bridge then registers
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a listener on the method signature and can provide an implementation.
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3- Renaming classes
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This simply changes the name of a class in its definition, as well as all its
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references in internal inner classes and methods.
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Calls from other classes are not modified -- they keep referencing the original
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class name. This allows the bridge to literally replace an implementation.
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An example will make this easier: android.graphics.Paint is the main drawing
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class that we need to replace. To do so, the generator renames Paint to _original_Paint.
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Later the bridge provides its own replacement version of Paint which will be used
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by the rest of the Android stack. The replacement version of Paint can still use
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(either by inheritance or delegation) all the original non-native code of _original_Paint
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if it so desires.
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Some of the Android classes are basically wrappers over native objects and since
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we don't have the native code in Eclipse, we need to provide a full alternate
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implementation. Sub-classing doesn't work as some native methods are static and
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we don't control object creation.
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This won't rename/replace the inner static methods of a given class.
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4- Method erasure based on return type
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This is mostly an implementation detail of the bridge: in the Paint class
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mentioned above, some inner static classes are used to pass around
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attributes (e.g. FontMetrics, or the Style enum) and all the original implementation
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is native.
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In this case we have a strategy that tells the generator that anything returning, for
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example, the inner class Paint$Style in the Paint class should be discarded and the
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bridge will provide its own implementation.
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--
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end
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