Update layoutlib create README [DO NOT MERGE]

- Description -

---------------

Layoutlib_create generates a JAR library used by the Eclipse graphical layout editor

to perform layout.

Layoutlib_create generates a JAR library used by the Eclipse graphical layout editor to perform

layout.

- Usage -

---------

 ./layoutlib_create path/to/android.jar destination.jar

 ./layoutlib_create destination.jar path/to/android1.jar path/to/android2.jar

- Design Overview -

-------------------

Layoutlib_create uses the "android.jar" containing all the Java code used by Android

as generated by the Android build, right before the classes are converted to a DEX format.

Layoutlib_create uses a few jars from the framework containing the Java code used by Android as

generated by the Android build, right before the classes are converted to a DEX format.

The Android JAR can't be used directly in Eclipse:

- it contains references to native code (which we want to avoid in Eclipse),

- some classes need to be overridden, for example all the drawing code that is

  replaced by Java 2D calls in Eclipse.

- some of the classes that need to be changed are final and/or we need access

  to their private internal state.

These jars can't be used directly in Eclipse as:

- they contains references to native code (which we want to avoid in Eclipse),

- some classes need to be overridden, for example all the drawing code that is replaced by Java 2D

  calls in Eclipse.

- some of the classes that need to be changed are final and/or we need access to their private

  internal state.

Consequently this tool:

- parses the input JAR,

- modifies some of the classes directly using some bytecode manipulation,

- filters some packages and removes those we don't want in the output JAR,

- injects some new classes,

- generates a modified JAR file that is suitable for the Android plugin

  for Eclipse to perform rendering.

- generates a modified JAR file that is suitable for the Android plugin for Eclipse to perform

  rendering.

The ASM library is used to do the bytecode modification using its visitor pattern API.

The layoutlib_create is *NOT* generic. There is no configuration file. Instead all the

configuration is done in the main() method and the CreateInfo structure is expected to

change with the Android platform as new classes are added, changed or removed.

The layoutlib_create is *NOT* generic. There is no configuration file. Instead all the configuration

is done in the main() method and the CreateInfo structure is expected to change with the Android

platform as new classes are added, changed or removed.

The resulting JAR is used by layoutlib_bridge (a.k.a. "the bridge"), also part of the

platform, that provides all the necessary missing implementation for rendering graphics

in Eclipse.

The resulting JAR is used by layoutlib_bridge (a.k.a. "the bridge"), also part of the platform, that

provides all the necessary missing implementation for rendering graphics in Eclipse.

- Analyzer

----------

The goal of the analyzer is to create a graph of all the classes from the input JAR

with their dependencies and then only keep the ones we want.

The goal of the analyzer is to create a graph of all the classes from the input JAR with their

dependencies and then only keep the ones we want.

To do that, the analyzer is created with a list of base classes to keep -- everything

that derives from these is kept. Currently the one such class is android.view.View:

since we want to render layouts, anything that is sort of a view needs to be kept.

To do that, the analyzer is created with a list of base classes to keep -- everything that derives

from these is kept. Currently the one such class is android.view.View: since we want to render

layouts, anything that is sort of a view needs to be kept.

The analyzer is also given a list of class names to keep in the output.

This is done using shell-like glob patterns that filter on the fully-qualified

class names, for example "android.*.R**" ("*" does not matches dots whilst "**" does,

and "." and "$" are interpreted as-is).

In practice we almost but not quite request the inclusion of full packages.

The analyzer is also given a list of class names to keep in the output. This is done using

shell-like glob patterns that filter on the fully-qualified class names, for example "android.*.R**"

("*" does not matches dots whilst "**" does, and "." and "$" are interpreted as-is). In practice we

almost but not quite request the inclusion of full packages.

The analyzer is also given a list of classes to exclude. A fake implementation of these

classes is injected by the Generator.

The analyzer is also given a list of classes to exclude. A fake implementation of these classes is

injected by the Generator.

With this information, the analyzer parses the input zip to find all the classes.

All classes deriving from the requested bases classes are kept.

All classes which name matched the glob pattern are kept.

The analysis then finds all the dependencies of the classes that are to be kept

using an ASM visitor on the class, the field types, the method types and annotations types.

Classes that belong to the current JRE are excluded.

With this information, the analyzer parses the input zip to find all the classes. All classes

deriving from the requested bases classes are kept. All classes whose name match the glob pattern

are kept. The analysis then finds all the dependencies of the classes that are to be kept using an

ASM visitor on the class, the field types, the method types and annotations types. Classes that

belong to the current JRE are excluded.

The output of the analyzer is a set of ASM ClassReader instances which are then

fed to the generator.

The output of the analyzer is a set of ASM ClassReader instances which are then fed to the

generator.

- Generator

-----------

The generator is constructed from a CreateInfo struct that acts as a config file

and lists:

- the classes to inject in the output JAR -- these classes are directly implemented

  in layoutlib_create and will be used to interface with the renderer in Eclipse.

The generator is constructed from a CreateInfo struct that acts as a config file and lists:

- the classes to inject in the output JAR -- these classes are directly implemented in

  layoutlib_create and will be used to interface with the renderer in Eclipse.

- specific methods to override (see method stubs details below).

- specific methods for which to delegate calls.

- specific methods to remove based on their return type.

- specific classes to rename.

- specific classes to refactor.

Each of these are specific strategies we use to be able to modify the Android code

to fit within the Eclipse renderer. These strategies are explained beow.

Each of these are specific strategies we use to be able to modify the Android code to fit within the

Eclipse renderer. These strategies are explained beow.

The core method of the generator is transform(): it takes an input ASM ClassReader

and modifies it to produce a byte array suitable for the final JAR file.

The core method of the generator is transform(): it takes an input ASM ClassReader and modifies it

to produce a byte array suitable for the final JAR file.

The first step of the transformation is to implement the method delegates.

The TransformClassAdapter is then used to process the potentially renamed class.

All protected or private classes are market as public.

All classes are made non-final.

Interfaces are left as-is.

The TransformClassAdapter is then used to process the potentially renamed class.  All protected or

private classes are market as public. All classes are made non-final. Interfaces are left as-is.

If a method has a return type that must be erased, the whole method is skipped.

Methods are also changed from protected/private to public.

The code of the methods is then kept as-is, except for native methods which are

replaced by a stub. Methods that are to be overridden are also replaced by a stub.

If a method has a return type that must be erased, the whole method is skipped.  Methods are also

changed from protected/private to public. The code of the methods is then kept as-is, except for

native methods which are replaced by a stub. Methods that are to be overridden are also replaced by

a stub.

Finally fields are also visited and changed from protected/private to public.

The next step of the transformation is changing the name of the class in case

we requested the class to be renamed. This uses the RenameClassAdapter to also rename

all inner classes and references in methods and types. Note that other classes are

not transformed and keep referencing the original name.

The class is then fed to RefactorClassAdapter which is like RenameClassAdapter but

updates the references in all classes. This is used to update the references of classes

in the java package that were added in the Dalvik VM but are not a part of the standard

JVM. The existing classes are modified to update all references to these non-standard

classes. An alternate implementation of these (com.android.tools.layoutlib.java.*) is

injected.

The ClassAdapters are chained together to achieve the desired output. (Look at section

2.2.7 Transformation chains in the asm user guide, link in the References.) The order of

execution of these is:

The next step of the transformation is changing the name of the class in case we requested the class

to be renamed. This uses the RenameClassAdapter to also rename all inner classes and references in

methods and types. Note that other classes are not transformed and keep referencing the original

name.

The class is then fed to RefactorClassAdapter which is like RenameClassAdapter but updates the

references in all classes. This is used to update the references of classes in the java package that

were added in the Dalvik VM but are not a part of the standard JVM. The existing classes are

modified to update all references to these non-standard classes. An alternate implementation of

these (com.android.tools.layoutlib.java.*) is injected.

RenameClassAdapter and RefactorClassAdapter both inherit from AbstractClassAdapter which changes the

class version (version of the JDK used to compile the class) to 50 (corresponding to Java 6), if the

class was originally compiled with Java 7 (version 51). This is because we don't currently generate

the StackMapTable correctly and Java 7 VM enforces that classes with version greater than 51 have

valid StackMapTable. As a side benefit of this, we can continue to support Java 6 because Java 7 on

Mac has horrible font rendering support.

The ClassAdapters are chained together to achieve the desired output. (Look at section 2.2.7

Transformation chains in the asm user guide, link in the References.) The order of execution of

these is:

ClassReader -> [DelegateClassAdapter] -> TransformClassAdapter -> [RenameClassAdapter] ->

RefactorClassAdapter -> ClassWriter

- Method stubs

--------------

As indicated above, all native and overridden methods are replaced by a stub.

We don't have the code to replace with in layoutlib_create.

Instead the StubMethodAdapter replaces the code of the method by a call to

OverrideMethod.invokeX(). When using the final JAR, the bridge can register

As indicated above, all native and overridden methods are replaced by a stub.  We don't have the

code to replace with in layoutlib_create. Instead the StubMethodAdapter replaces the code of the

method by a call to OverrideMethod.invokeX(). When using the final JAR, the bridge can register

listeners from these overridden method calls based on the method signatures.

The listeners are currently pretty basic: we only pass the signature of the

method being called, its caller object and a flag indicating whether the

method was native. We do not currently provide the parameters. The listener

can however specify the return value of the overridden method.

The listeners are currently pretty basic: we only pass the signature of the method being called, its

caller object and a flag indicating whether the method was native. We do not currently provide the

parameters. The listener can however specify the return value of the overridden method.

This strategy is now obsolete and replaced by the method delegates.

- Strategies

------------

We currently have 6 strategies to deal with overriding the rendering code

and make it run in Eclipse. Most of these strategies are implemented hand-in-hand

by the bridge (which runs in Eclipse) and the generator.

We currently have 6 strategies to deal with overriding the rendering code and make it run in

Eclipse. Most of these strategies are implemented hand-in-hand by the bridge (which runs in Eclipse)

and the generator.

1- Class Injection

This is the easiest: we currently inject the following classes:

- OverrideMethod and its associated MethodListener and MethodAdapter are used

  to intercept calls to some specific methods that are stubbed out and change

  their return value.

- CreateInfo class, which configured the generator. Not used yet, but could

  in theory help us track what the generator changed.

- AutoCloseable and Objects are part of Java 7. To enable us to still run on Java 6, new

  classes are injected. The implementation for these classes has been taken from

  Android's libcore (platform/libcore/luni/src/main/java/java/...).

- Charsets, IntegralToString and UnsafeByteSequence are not part of the standard JAVA VM.

  They are added to the Dalvik VM for performance reasons. An implementation that is very

  close to the original (which is at platform/libcore/luni/src/main/java/...) is injected.

  Since these classees were in part of the java package, where we can't inject classes,

  all references to these have been updated (See strategy 4- Refactoring Classes).

- OverrideMethod and its associated MethodListener and MethodAdapter are used to intercept calls to

  some specific methods that are stubbed out and change their return value.

- CreateInfo class, which configured the generator. Not used yet, but could in theory help us track

  what the generator changed.

- AutoCloseable and Objects are part of Java 7. To enable us to still run on Java 6, new classes are

  injected. The implementation for these classes has been taken from Android's libcore

  (platform/libcore/luni/src/main/java/java/...).

- Charsets, IntegralToString and UnsafeByteSequence are not part of the standard JAVA VM. They are

  added to the Dalvik VM for performance reasons. An implementation that is very close to the

  original (which is at platform/libcore/luni/src/main/java/...) is injected. Since these classees

  were in part of the java package, where we can't inject classes, all references to these have been

  updated (See strategy 4- Refactoring Classes).

2- Overriding methods

As explained earlier, the creator doesn't have any replacement code for

methods to override. Instead it removes the original code and replaces it

by a call to a specific OveriddeMethod.invokeX(). The bridge then registers

a listener on the method signature and can provide an implementation.

As explained earlier, the creator doesn't have any replacement code for methods to override. Instead

it removes the original code and replaces it by a call to a specific OveriddeMethod.invokeX(). The

bridge then registers a listener on the method signature and can provide an implementation.

This strategy is now obsolete and replaced by the method delegates.

See strategy 5 below.

This strategy is now obsolete and replaced by the method delegates (See strategy 6- Method

Delegates).

3- Renaming classes

This simply changes the name of a class in its definition, as well as all its

references in internal inner classes and methods.

Calls from other classes are not modified -- they keep referencing the original

class name. This allows the bridge to literally replace an implementation.

This simply changes the name of a class in its definition, as well as all its references in internal

inner classes and methods. Calls from other classes are not modified -- they keep referencing the

original class name. This allows the bridge to literally replace an implementation.

An example will make this easier: android.graphics.Paint is the main drawing

class that we need to replace. To do so, the generator renames Paint to _original_Paint.

Later the bridge provides its own replacement version of Paint which will be used

by the rest of the Android stack. The replacement version of Paint can still use

(either by inheritance or delegation) all the original non-native code of _original_Paint

if it so desires.

An example will make this easier: android.graphics.Paint is the main drawing class that we need to

replace. To do so, the generator renames Paint to _original_Paint. Later the bridge provides its own

replacement version of Paint which will be used by the rest of the Android stack. The replacement

version of Paint can still use (either by inheritance or delegation) all the original non-native

code of _original_Paint if it so desires.

Some of the Android classes are basically wrappers over native objects and since

we don't have the native code in Eclipse, we need to provide a full alternate

implementation. Sub-classing doesn't work as some native methods are static and

we don't control object creation.

Some of the Android classes are basically wrappers over native objects and since we don't have the

native code in Eclipse, we need to provide a full alternate implementation. Sub-classing doesn't

work as some native methods are static and we don't control object creation.

This won't rename/replace the inner static methods of a given class.

4- Refactoring classes

This is very similar to the Renaming classes except that it also updates the reference in

all classes. This is done for classes which are added to the Dalvik VM for performance

reasons but are not present in the Standard Java VM. An implementation for these classes

is also injected.

This is very similar to the Renaming classes except that it also updates the reference in all

classes. This is done for classes which are added to the Dalvik VM for performance reasons but are

not present in the Standard Java VM. An implementation for these classes is also injected.

5- Method erasure based on return type

This is mostly an implementation detail of the bridge: in the Paint class

mentioned above, some inner static classes are used to pass around

attributes (e.g. FontMetrics, or the Style enum) and all the original implementation

is native.

This is mostly an implementation detail of the bridge: in the Paint class mentioned above, some

inner static classes are used to pass around attributes (e.g. FontMetrics, or the Style enum) and

all the original implementation is native.

In this case we have a strategy that tells the generator that anything returning, for

example, the inner class Paint$Style in the Paint class should be discarded and the

bridge will provide its own implementation.

In this case we have a strategy that tells the generator that anything returning, for example, the

inner class Paint$Style in the Paint class should be discarded and the bridge will provide its own

implementation.

6- Method Delegates

This strategy is used to override method implementations.

Given a method SomeClass.MethodName(), 1 or 2 methods are generated:

a- A copy of the original method named SomeClass.MethodName_Original().

   The content is the original method as-is from the reader.

   This step is omitted if the method is native, since it has no Java implementation.

b- A brand new implementation of SomeClass.MethodName() which calls to a

   non-existing static method named SomeClass_Delegate.MethodName().

   The implementation of this 'delegate' method is done in layoutlib_brigde.

The delegate method is a static method.

If the original method is non-static, the delegate method receives the original 'this'

as its first argument. If the original method is an inner non-static method, it also

receives the inner 'this' as the second argument.

This strategy is used to override method implementations. Given a method SomeClass.MethodName(), 1

or 2 methods are generated:

a- A copy of the original method named SomeClass.MethodName_Original(). The content is the original

method as-is from the reader. This step is omitted if the method is native, since it has no Java

implementation.

b- A brand new implementation of SomeClass.MethodName() which calls to a non-existing static method

named SomeClass_Delegate.MethodName(). The implementation of this 'delegate' method is done in

layoutlib_brigde.

The delegate method is a static method. If the original method is non-static, the delegate method

receives the original 'this' as its first argument. If the original method is an inner non-static

method, it also receives the inner 'this' as the second argument.