1 # Copyright (C) 2008 The Android Open Source Project 2 3 4 - Description - 5 --------------- 6 7 Layoutlib_create generates a JAR library used by the Eclipse graphical layout editor to perform 8 layout. 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 20 Layoutlib_create uses a few jars from the framework containing the Java code used by Android as 21 generated by the Android build, right before the classes are converted to a DEX format. 22 23 These 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 30 Consequently 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 38 The ASM library is used to do the bytecode modification using its visitor pattern API. 39 40 The layoutlib_create is *NOT* generic. There is no configuration file. Instead all the configuration 41 is done in the main() method and the CreateInfo structure is expected to change with the Android 42 platform as new classes are added, changed or removed. Some configuration that may be platform 43 dependent is also present elsewhere in code. 44 45 The resulting JAR is used by layoutlib_bridge (a.k.a. "the bridge"), also part of the platform, that 46 provides all the necessary missing implementation for rendering graphics in Eclipse. 47 48 49 50 - Implementation Notes - 51 ------------------------ 52 53 The 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 61 The goal of the analyzer is to create a graph of all the classes from the input JAR with their 62 dependencies and then only keep the ones we want. 63 64 To do that, the analyzer is created with a list of base classes to keep -- everything that derives 65 from these is kept. Currently the one such class is android.view.View: since we want to render 66 layouts, anything that is sort of a view needs to be kept. 67 68 The analyzer is also given a list of class names to keep in the output. This is done using 69 shell-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 71 almost but not quite request the inclusion of full packages. 72 73 The analyzer is also given a list of classes to exclude. A fake implementation of these classes is 74 injected by the Generator. 75 76 With this information, the analyzer parses the input zip to find all the classes. All classes 77 deriving from the requested bases classes are kept. All classes whose name match the glob pattern 78 are kept. The analysis then finds all the dependencies of the classes that are to be kept using an 79 ASM visitor on the class, the field types, the method types and annotations types. Classes that 80 belong to the current JRE are excluded. 81 82 The output of the analyzer is a set of ASM ClassReader instances which are then fed to the 83 generator. 84 85 86 - Generator 87 ----------- 88 89 The 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 98 Each of these are specific strategies we use to be able to modify the Android code to fit within the 99 Eclipse renderer. These strategies are explained below. 100 101 The core method of the generator is transform(): it takes an input ASM ClassReader and modifies it 102 to produce a byte array suitable for the final JAR file. 103 104 The first step of the transformation is to implement the method delegates. 105 106 The TransformClassAdapter is then used to process the potentially renamed class. All protected or 107 private classes are market as public. All classes are made non-final. Interfaces are left as-is. 108 109 If a method has a return type that must be erased, the whole method is skipped. Methods are also 110 changed from protected/private to public. The code of the methods is then kept as-is, except for 111 native methods which are replaced by a stub. Methods that are to be overridden are also replaced by 112 a stub. 113 114 Finally fields are also visited and changed from protected/private to public. 115 116 The next step of the transformation is changing the name of the class in case we requested the class 117 to be renamed. This uses the RenameClassAdapter to also rename all inner classes and references in 118 methods and types. Note that other classes are not transformed and keep referencing the original 119 name. 120 121 The class is then fed to RefactorClassAdapter which is like RenameClassAdapter but updates the 122 references in all classes. This is used to update the references of classes in the java package that 123 were added in the Dalvik VM but are not a part of the Desktop VM. The existing classes are 124 modified to update all references to these non-desktop classes. An alternate implementation of 125 these (com.android.tools.layoutlib.java.*) is injected. 126 127 RenameClassAdapter and RefactorClassAdapter both inherit from AbstractClassAdapter which changes the 128 class version (version of the JDK used to compile the class) to 50 (corresponding to Java 6), if the 129 class was originally compiled with Java 7 (version 51). This is because we don't currently generate 130 the StackMapTable correctly and Java 7 VM enforces that classes with version greater than 51 have 131 valid StackMapTable. As a side benefit of this, we can continue to support Java 6 because Java 7 on 132 Mac has horrible font rendering support. 133 134 ReplaceMethodCallsAdapter replaces calls to certain methods. This is different from the 135 DelegateMethodAdapter since it doesn't preserve the original copy of the method and more importantly 136 changes the calls to a method in each class instead of changing the implementation of the method. 137 This is useful for methods in the Java namespace where we cannot add delegates. The configuration 138 for this is not done through the CreateInfo class, but done in the ReplaceMethodAdapter. 139 140 The ClassAdapters are chained together to achieve the desired output. (Look at section 2.2.7 141 Transformation chains in the asm user guide, link in the References.) The order of execution of 142 these is: 143 ClassReader -> [DelegateClassAdapter] -> TransformClassAdapter -> [RenameClassAdapter] -> 144 RefactorClassAdapter -> [ReplaceMethodCallsAdapter] -> ClassWriter 145 146 - Method stubs 147 -------------- 148 149 As indicated above, all native and overridden methods are replaced by a stub. We don't have the 150 code to replace with in layoutlib_create. Instead the StubMethodAdapter replaces the code of the 151 method by a call to OverrideMethod.invokeX(). When using the final JAR, the bridge can register 152 listeners from these overridden method calls based on the method signatures. 153 154 The listeners are currently pretty basic: we only pass the signature of the method being called, its 155 caller object and a flag indicating whether the method was native. We do not currently provide the 156 parameters. The listener can however specify the return value of the overridden method. 157 158 This strategy is now obsolete and replaced by the method delegates. 159 160 161 - Strategies 162 ------------ 163 164 We currently have 6 strategies to deal with overriding the rendering code and make it run in 165 Eclipse. Most of these strategies are implemented hand-in-hand by the bridge (which runs in Eclipse) 166 and the generator. 167 168 169 1- Class Injection 170 171 This 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). 184 185 186 2- Overriding methods 187 188 As explained earlier, the creator doesn't have any replacement code for methods to override. Instead 189 it removes the original code and replaces it by a call to a specific OveriddeMethod.invokeX(). The 190 bridge then registers a listener on the method signature and can provide an implementation. 191 192 This strategy is now obsolete and replaced by the method delegates (See strategy 6- Method 193 Delegates). 194 195 196 3- Renaming classes 197 198 This simply changes the name of a class in its definition, as well as all its references in internal 199 inner classes and methods. Calls from other classes are not modified -- they keep referencing the 200 original class name. This allows the bridge to literally replace an implementation. 201 202 An example will make this easier: android.graphics.Paint is the main drawing class that we need to 203 replace. To do so, the generator renames Paint to _original_Paint. Later the bridge provides its own 204 replacement version of Paint which will be used by the rest of the Android stack. The replacement 205 version of Paint can still use (either by inheritance or delegation) all the original non-native 206 code of _original_Paint if it so desires. 207 208 Some of the Android classes are basically wrappers over native objects and since we don't have the 209 native code in Eclipse, we need to provide a full alternate implementation. Sub-classing doesn't 210 work as some native methods are static and we don't control object creation. 211 212 This won't rename/replace the inner static methods of a given class. 213 214 215 4- Refactoring classes 216 217 This is very similar to the Renaming classes except that it also updates the reference in all 218 classes. This is done for classes which are added to the Dalvik VM for performance reasons but are 219 not present in the Desktop VM. An implementation for these classes is also injected. 220 221 222 5- Method erasure based on return type 223 224 This is mostly an implementation detail of the bridge: in the Paint class mentioned above, some 225 inner static classes are used to pass around attributes (e.g. FontMetrics, or the Style enum) and 226 all the original implementation is native. 227 228 In this case we have a strategy that tells the generator that anything returning, for example, the 229 inner class Paint$Style in the Paint class should be discarded and the bridge will provide its own 230 implementation. 231 232 233 6- Method Delegates 234 235 This strategy is used to override method implementations. Given a method SomeClass.MethodName(), 1 236 or 2 methods are generated: 237 a- A copy of the original method named SomeClass.MethodName_Original(). The content is the original 238 method as-is from the reader. This step is omitted if the method is native, since it has no Java 239 implementation. 240 b- A brand new implementation of SomeClass.MethodName() which calls to a non-existing static method 241 named SomeClass_Delegate.MethodName(). The implementation of this 'delegate' method is done in 242 layoutlib_bridge. 243 244 The delegate method is a static method. If the original method is non-static, the delegate method 245 receives the original 'this' as its first argument. If the original method is an inner non-static 246 method, it also receives the inner 'this' as the second argument. 247 248 249 250 - References - 251 -------------- 252 253 254 The JVM Specification 2nd edition: 255 http://java.sun.com/docs/books/jvms/second_edition/html/VMSpecTOC.doc.html 256 257 Understanding bytecode: 258 http://www.ibm.com/developerworks/ibm/library/it-haggar_bytecode/ 259 260 Bytecode opcode list: 261 http://en.wikipedia.org/wiki/Java_bytecode_instruction_listings 262 263 ASM user guide: 264 http://download.forge.objectweb.org/asm/asm4-guide.pdf 265 266 267 -- 268 end 269
