Booster

Library Instrumentation

What Is Library Instrumentation?

Typically, we modify or inject bytecode in ClassTransformer. What if the bytecode injected in ClassTransformer depends on another library (JAR / AAR)? There are several approaches:

  1. Embed the library in resources, then copy it to the Transform output directory in ClassTransformer
  2. Download from remote to the Transform output directory
  3. ......

What if this library (JAR / AAR) depends on other libraries (JAR / AAR)? Although this makes things more complicated, it's not impossible. We can integrate dependency management tools like Maven or Ivy.

What if a transitive dependency library already exists in the APP, but the version differs from what the APP depends on? In this case, the previous methods become difficult to implement. Is there a simpler approach? Of course there is - this is the original intention behind designing VariantProcessor, to make large-scale bytecode injection easier.

How It Works?

First, let's review the respective responsibilities of ClassTransformer and VariantProcessor:

  • ClassTransformer is mainly used for bytecode manipulation. It's not easy for ClassTransformer to handle content other than bytecode, such as resources, creating Task, etc.
  • VariantProcessor is mainly responsible for work other than bytecode manipulation, such as creating Task, accessing build intermediate artifacts, etc.

Why design it this way? There are mainly two considerations:

  1. Clearer division of responsibilities
  2. Decoupling ClassTransformer from Gradle API

On one hand, this facilitates unit testing; on the other hand, it allows ClassTransformer to be reused in non-Gradle projects.

Practice

Many Android developers have encountered dynamic library loading failures, for example:

java.lang.UnsatisfiedLinkError: Couldn't load xxx from loader dalvik.system.PathClassLoader: findLibrary returned null
  at java.lang.Runtime.loadLibrary(Runtime.java:365)
  at java.lang.System.loadLibrary(System.java:535)
  at com.your.app.NativeClass.<clinit>(Native.java:16)
  ... 63 more
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We can use ReLinkeropen in new window to prevent such crashes. How to use Booster to inject ReLinkeropen in new window? This question is left as an exercise for the readers.