Android动态类生成预加载-dexmaker使用

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一、dexmaker简单介绍

dexmaker是运行在Android Dalvik VM上，利用Java编写，来动态生成DEX字节码的API。如果读者了解AOP编程的话，应该听说过cglib or ASM，但这两个工具生成都是Java字节码，而Dalvik加载的必须是DEX字节码。所以，想要在Android上进行AOP编程，dexmaker可以说是一个很好的选择。项目地址：https://github.com/crittercism/dexmaker。

二。简单使用

下面这个例子非常典型，可以说入门非常好了。过程很简单，生成一个包含一个函数的类，在主程序里面动态加载（使用ClassLoader），然后执行类里面的函数。这是在Java平台的例子，我直接在Android上进行编程的，后面或说明相应的问题以及解决办法，下来看看这个例子吧。

public final class HelloWorldMaker {
public static void main(String[] args) throws Exception {
DexMaker dexMaker = new DexMaker(); // Generate a HelloWorld class.
TypeId<?> helloWorld = TypeId.get("LHelloWorld;");
dexMaker.declare(helloWorld, "HelloWorld.generated", Modifier.PUBLIC, TypeId.OBJECT);
generateHelloMethod(dexMaker, helloWorld); // Create the dex file and load it.
File outputDir = new File(".");
ClassLoader loader = dexMaker.generateAndLoad(HelloWorldMaker.class.getClassLoader(),
outputDir, outputDir);
Class<?> helloWorldClass = loader.loadClass("HelloWorld"); // Execute our newly-generated code in-process.
helloWorldClass.getMethod("hello").invoke(null);
} private static void generateHelloMethod(DexMaker dexMaker, TypeId<?> declaringType) {
// Lookup some types we'll need along the way.
TypeId<System> systemType = TypeId.get(System.class);
TypeId<PrintStream> printStreamType = TypeId.get(PrintStream.class); // Identify the 'hello()' method on declaringType.
MethodId hello = declaringType.getMethod(TypeId.VOID, "hello"); // Declare that method on the dexMaker. Use the returned Code instance
// as a builder that we can append instructions to.
Code code = dexMaker.declare(hello, Modifier.STATIC | Modifier.PUBLIC); // Declare all the locals we'll need up front. The API requires this.
Local<Integer> a = code.newLocal(TypeId.INT);
Local<Integer> b = code.newLocal(TypeId.INT);
Local<Integer> c = code.newLocal(TypeId.INT);
Local<String> s = code.newLocal(TypeId.STRING);
Local<PrintStream> localSystemOut = code.newLocal(printStreamType); // int a = 0xabcd;
code.loadConstant(a, 0xabcd); // int b = 0xaaaa;
code.loadConstant(b, 0xaaaa); // int c = a - b;
code.op(BinaryOp.SUBTRACT, c, a, b); // String s = Integer.toHexString(c);
MethodId<Integer, String> toHexString
= TypeId.get(Integer.class).getMethod(TypeId.STRING, "toHexString", TypeId.INT);
code.invokeStatic(toHexString, s, c); // System.out.println(s);
FieldId<System, PrintStream> systemOutField = systemType.getField(printStreamType, "out");
code.sget(systemOutField, localSystemOut);
MethodId<PrintStream, Void> printlnMethod = printStreamType.getMethod(
TypeId.VOID, "println", TypeId.STRING);
code.invokeVirtual(printlnMethod, null, localSystemOut, s); // return;
code.returnVoid();
}
}