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README.txt

1# Copyright (C) 2008 The Android Open Source Project
2
3
4- Description -
5---------------
6
7Layoutlib_create generates a JAR library used by the Eclipse graphical layout editor to perform
8layout.
9
10
11- Usage -
12---------
13
14 ./layoutlib_create destination.jar path/to/android1.jar path/to/android2.jar
15
16
17- Design Overview -
18-------------------
19
20Layoutlib_create uses a few jars from the framework containing the Java code used by Android as
21generated by the Android build, right before the classes are converted to a DEX format.
22
23These jars can't be used directly in Eclipse as:
24- they contains references to native code (which we want to avoid in Eclipse),
25- some classes need to be overridden, for example all the drawing code that is replaced by Java 2D
26  calls in Eclipse.
27- some of the classes that need to be changed are final and/or we need access to their private
28  internal state.
29
30Consequently this tool:
31- parses the input JAR,
32- modifies some of the classes directly using some bytecode manipulation,
33- filters some packages and removes those we don't want in the output JAR,
34- injects some new classes,
35- generates a modified JAR file that is suitable for the Android plugin for Eclipse to perform
36  rendering.
37
38The ASM library is used to do the bytecode modification using its visitor pattern API.
39
40The layoutlib_create is *NOT* generic. There is no configuration file. Instead all the configuration
41is done in the main() method and the CreateInfo structure is expected to change with the Android
42platform as new classes are added, changed or removed. Some configuration that may be platform
43dependent is also present elsewhere in code.
44
45The resulting JAR is used by layoutlib_bridge (a.k.a. "the bridge"), also part of the platform, that
46provides all the necessary missing implementation for rendering graphics in Eclipse.
47
48
49
50- Implementation Notes -
51------------------------
52
53The tool works in two phases:
54- first analyze the input jar (AsmAnalyzer class)
55- then generate the output jar (AsmGenerator class),
56
57
58- Analyzer
59----------
60
61The goal of the analyzer is to create a graph of all the classes from the input JAR with their
62dependencies and then only keep the ones we want.
63
64To do that, the analyzer is created with a list of base classes to keep -- everything that derives
65from these is kept. Currently the one such class is android.view.View: since we want to render
66layouts, anything that is sort of a view needs to be kept.
67
68The analyzer is also given a list of class names to keep in the output. This is done using
69shell-like glob patterns that filter on the fully-qualified class names, for example "android.*.R**"
70("*" does not matches dots whilst "**" does, and "." and "$" are interpreted as-is). In practice we
71almost but not quite request the inclusion of full packages.
72
73The analyzer is also given a list of classes to exclude. A fake implementation of these classes is
74injected by the Generator.
75
76With this information, the analyzer parses the input zip to find all the classes. All classes
77deriving from the requested bases classes are kept. All classes whose name match the glob pattern
78are kept. The analysis then finds all the dependencies of the classes that are to be kept using an
79ASM visitor on the class, the field types, the method types and annotations types. Classes that
80belong to the current JRE are excluded.
81
82The output of the analyzer is a set of ASM ClassReader instances which are then fed to the
83generator.
84
85
86- Generator
87-----------
88
89The generator is constructed from a CreateInfo struct that acts as a config file and lists:
90- the classes to inject in the output JAR -- these classes are directly implemented in
91  layoutlib_create and will be used to interface with the renderer in Eclipse.
92- specific methods to override (see method stubs details below).
93- specific methods for which to delegate calls.
94- specific methods to remove based on their return type.
95- specific classes to rename.
96- specific classes to refactor.
97
98Each of these are specific strategies we use to be able to modify the Android code to fit within the
99Eclipse renderer. These strategies are explained below.
100
101The core method of the generator is transform(): it takes an input ASM ClassReader and modifies it
102to produce a byte array suitable for the final JAR file.
103
104The first step of the transformation is to implement the method delegates.
105
106The TransformClassAdapter is then used to process the potentially renamed class.  All protected or
107private classes are market as public. All classes are made non-final. Interfaces are left as-is.
108
109If a method has a return type that must be erased, the whole method is skipped.  Methods are also
110changed from protected/private to public. The code of the methods is then kept as-is, except for
111native methods which are replaced by a stub. Methods that are to be overridden are also replaced by
112a stub.
113
114Finally fields are also visited and changed from protected/private to public.
115
116The next step of the transformation is changing the name of the class in case we requested the class
117to be renamed. This uses the RenameClassAdapter to also rename all inner classes and references in
118methods and types. Note that other classes are not transformed and keep referencing the original
119name.
120
121The class is then fed to RefactorClassAdapter which is like RenameClassAdapter but updates the
122references in all classes. This is used to update the references of classes in the java package that
123were added in the Dalvik VM but are not a part of the Desktop VM. The existing classes are
124modified to update all references to these non-desktop classes. An alternate implementation of
125these (com.android.tools.layoutlib.java.*) is injected.
126
127RenameClassAdapter and RefactorClassAdapter both inherit from AbstractClassAdapter which changes the
128class version (version of the JDK used to compile the class) to 50 (corresponding to Java 6), if the
129class was originally compiled with Java 7 (version 51). This is because we don't currently generate
130the StackMapTable correctly and Java 7 VM enforces that classes with version greater than 51 have
131valid StackMapTable. As a side benefit of this, we can continue to support Java 6 because Java 7 on
132Mac has horrible font rendering support.
133
134ReplaceMethodCallsAdapter replaces calls to certain methods. This is different from the
135DelegateMethodAdapter since it doesn't preserve the original copy of the method and more importantly
136changes the calls to a method in each class instead of changing the implementation of the method.
137This is useful for methods in the Java namespace where we cannot add delegates. The configuration
138for this is not done through the CreateInfo class, but done in the ReplaceMethodAdapter.
139
140The ClassAdapters are chained together to achieve the desired output. (Look at section 2.2.7
141Transformation chains in the asm user guide, link in the References.) The order of execution of
142these is:
143ClassReader -> [DelegateClassAdapter] -> TransformClassAdapter -> [RenameClassAdapter] ->
144RefactorClassAdapter -> [ReplaceMethodCallsAdapter] -> ClassWriter
145
146- Method stubs
147--------------
148
149As indicated above, all native and overridden methods are replaced by a stub.  We don't have the
150code to replace with in layoutlib_create. Instead the StubMethodAdapter replaces the code of the
151method by a call to OverrideMethod.invokeX(). When using the final JAR, the bridge can register
152listeners from these overridden method calls based on the method signatures.
153
154The listeners are currently pretty basic: we only pass the signature of the method being called, its
155caller object and a flag indicating whether the method was native. We do not currently provide the
156parameters. The listener can however specify the return value of the overridden method.
157
158This strategy is now obsolete and replaced by the method delegates.
159
160
161- Strategies
162------------
163
164We currently have 6 strategies to deal with overriding the rendering code and make it run in
165Eclipse. Most of these strategies are implemented hand-in-hand by the bridge (which runs in Eclipse)
166and the generator.
167
168
1691- Class Injection
170
171This is the easiest: we currently inject the following classes:
172- OverrideMethod and its associated MethodListener and MethodAdapter are used to intercept calls to
173  some specific methods that are stubbed out and change their return value.
174- CreateInfo class, which configured the generator. Not used yet, but could in theory help us track
175  what the generator changed.
176- AutoCloseable and Objects are part of Java 7. To enable us to still run on Java 6, new classes are
177  injected. The implementation for these classes has been taken from Android's libcore
178  (platform/libcore/luni/src/main/java/java/...).
179- Charsets, IntegralToString and UnsafeByteSequence are not part of the Desktop VM. They are
180  added to the Dalvik VM for performance reasons. An implementation that is very close to the
181  original (which is at platform/libcore/luni/src/main/java/...) is injected. Since these classees
182  were in part of the java package, where we can't inject classes, all references to these have been
183  updated (See strategy 4- Refactoring Classes).
184
185
1862- Overriding methods
187
188As explained earlier, the creator doesn't have any replacement code for methods to override. Instead
189it removes the original code and replaces it by a call to a specific OveriddeMethod.invokeX(). The
190bridge then registers a listener on the method signature and can provide an implementation.
191
192This strategy is now obsolete and replaced by the method delegates (See strategy 6- Method
193Delegates).
194
195
1963- Renaming classes
197
198This simply changes the name of a class in its definition, as well as all its references in internal
199inner classes and methods. Calls from other classes are not modified -- they keep referencing the
200original class name. This allows the bridge to literally replace an implementation.
201
202An example will make this easier: android.graphics.Paint is the main drawing class that we need to
203replace. To do so, the generator renames Paint to _original_Paint. Later the bridge provides its own
204replacement version of Paint which will be used by the rest of the Android stack. The replacement
205version of Paint can still use (either by inheritance or delegation) all the original non-native
206code of _original_Paint if it so desires.
207
208Some of the Android classes are basically wrappers over native objects and since we don't have the
209native code in Eclipse, we need to provide a full alternate implementation. Sub-classing doesn't
210work as some native methods are static and we don't control object creation.
211
212This won't rename/replace the inner static methods of a given class.
213
214
2154- Refactoring classes
216
217This is very similar to the Renaming classes except that it also updates the reference in all
218classes. This is done for classes which are added to the Dalvik VM for performance reasons but are
219not present in the Desktop VM. An implementation for these classes is also injected.
220
221
2225- Method erasure based on return type
223
224This is mostly an implementation detail of the bridge: in the Paint class mentioned above, some
225inner static classes are used to pass around attributes (e.g. FontMetrics, or the Style enum) and
226all the original implementation is native.
227
228In this case we have a strategy that tells the generator that anything returning, for example, the
229inner class Paint$Style in the Paint class should be discarded and the bridge will provide its own
230implementation.
231
232
2336- Method Delegates
234
235This strategy is used to override method implementations. Given a method SomeClass.MethodName(), 1
236or 2 methods are generated:
237a- A copy of the original method named SomeClass.MethodName_Original(). The content is the original
238method as-is from the reader. This step is omitted if the method is native, since it has no Java
239implementation.
240b- A brand new implementation of SomeClass.MethodName() which calls to a non-existing static method
241named SomeClass_Delegate.MethodName(). The implementation of this 'delegate' method is done in
242layoutlib_bridge.
243
244The delegate method is a static method. If the original method is non-static, the delegate method
245receives the original 'this' as its first argument. If the original method is an inner non-static
246method, it also receives the inner 'this' as the second argument.
247
248
249
250- References -
251--------------
252
253
254The JVM Specification 2nd edition:
255  http://java.sun.com/docs/books/jvms/second_edition/html/VMSpecTOC.doc.html
256
257Understanding bytecode:
258  http://www.ibm.com/developerworks/ibm/library/it-haggar_bytecode/
259
260Bytecode opcode list:
261  http://en.wikipedia.org/wiki/Java_bytecode_instruction_listings
262
263ASM user guide:
264  http://download.forge.objectweb.org/asm/asm4-guide.pdf
265
266
267--
268end
269