準備技術:

1. ASM
2. Class Loader
3. Reflection
4. IOC/DI

當Java Class進入到JVM，是以Bytecode格式進入，ASM的目的是希望能夠修改Bytecode或動態產生Class。為什麼要修改Class?因為有這樣的機制，我們可以動態的將Clas加料，這是依賴注入。

如圖，針對Spring Container我們可以注入MetaData和POJO資料改變一些Class的結果，達到另一個功能實現的目的。

1. 先看一個例子，我們想要寫一個Class觀看另一個Class的變數。變數有Static的，final的，以及非Static或Final的如String，還有double。

   public class ForReadClass {

    final int init = 110;
    private final Integer intField = 120;
    public final String stringField = "Public Final Strng Value";
    public static String commStr = "Common String value";
    String str = "Just a string value";
    final double d = 1.1;
    final Double D = 1.2;
   
    public ForReadClass() {
    }
   
    public void methodA() {
    	System.out.println(intField);
    }
   
    @SuppressWarnings("unchecked")
    public static void main(String[] args) {
   
    	  try { ClassReader reader = new ClassReader("core.ForReadClass");
    	  		ClassNode cn = new ClassNode(); reader.accept(cn, 0);
    	  			System.out.println(cn.name); List<FieldNode> fieldList = cn.fields;
    	  			for (FieldNode fieldNode : fieldList) {
    	  					System.out.println("Field name: " + fieldNode.name);
    	  					System.out.println("Field desc: " + fieldNode.desc);
    	  					System.out.println("Filed value: " + fieldNode.value);
    	  					System.out.println("Filed access: " + fieldNode.access); } 
    	  	} catch(IOException e) 
    	  			{ e.printStackTrace(); } 
    	  }
   

   }

2. 程式執行之後，很明顯的，沒有抓到 非Static或Final系列的變數。

3. 利用ASM的AbstractInNode方式抓到全部變數而看到，但是經由測試，Double變數沒抓到，這以後修改。

    public static void main(String[] args) {
   
   
    	try {
    		ClassReader reader = new ClassReader("core.ForReadClass");
    		ClassNode cn = new ClassNode();
    		reader.accept(cn, 0);
    		List<MethodNode> methodList = cn.methods;
    		for (MethodNode md : methodList) {
    			System.out.println(md.name);
    			System.out.println(md.access);
    			System.out.println(md.desc);
    			System.out.println(md.signature);
    			List<LocalVariableNode> lvNodeList = md.localVariables;
    			for (LocalVariableNode lvn : lvNodeList) {
    				System.out.println("Local name: " + lvn.name);
    				System.out.println("Local name: " + lvn.start.getLabel());
    				System.out.println("Local name: " + lvn.desc);
    				System.out.println("Local name: " + lvn.signature);
    			}
    			Iterator<AbstractInsnNode> instraIter = md.instructions.iterator();
    			while (instraIter.hasNext()) {
    				AbstractInsnNode abi = instraIter.next();
    				if (abi instanceof LdcInsnNode) {
    					LdcInsnNode ldcI = (LdcInsnNode) abi;
    					System.out.println("LDC node value: " + ldcI.cst);
    				}
    			}
    		}
    		MethodVisitor mv = cn.visitMethod(Opcodes.AALOAD, "<init>", Type
    				.getType(String.class).toString(), null, null);
    		mv.visitFieldInsn(Opcodes.GETFIELD,
    				Type.getInternalName(String.class), "str",
    				Type.getType(String.class).toString());
    		System.out.println(cn.name);
    		List<FieldNode> fieldList = cn.fields;
    		for (FieldNode fieldNode : fieldList) {
    			System.out.println("Field name: " + fieldNode.name);
    			System.out.println("Field desc: " + fieldNode.desc);
    			System.out.println("Filed value: " + fieldNode.value);
    			System.out.println("Filed access: " + fieldNode.access);
    			/*
    			if (fieldNode.visibleAnnotations != null) {
    				for (AnnotationNode	 anNode : 	fieldNode.visibleAnnotations) {
                        System.out.println(anNode.desc);
                    }
    			}
    			*/
    		}
    	} catch (IOException e) {
    		e.printStackTrace();
    	} catch (SecurityException e) {
    		e.printStackTrace();
    	} catch (IllegalArgumentException e) {
    		e.printStackTrace();
    	}
    }
   

   }

4. 

參考書籍:

1. ASM官網
2. Spring in IOC/DI

Day 24 結束