#yyds干货盘点#Java ASM系列（（097）生成Control Flow Graph） _干货

采得百花成蜜后，为谁辛苦为谁甜。这篇文章主要讲述#yyds干货盘点#Java ASM系列：（097）生成Control Flow Graph相关的知识，希望能为你提供帮助。
本文属于Java ASM系列三：Tree API当中的一篇。
1. 如何生成控制流程图 1.1. 整体思路

Instruction --> Block --> Box（图形） --> Graph（图形）

【#yyds干货盘点#Java ASM系列（（097）生成Control Flow Graph）】生成控制流程图经过三个步骤：

第一步，将Instruction组织成Block。在每个Block当中，包含1个或多个Instruction。
第二步，将Block封装成Box。Block就是单纯的Instruction的有序集合，而Box在Block的基础上添加了Rectangle图形信息。
第三步，将多个Box组织到一起，形成一个整体的Graph。

文章图片

1.2. 难点解析
在这个过程中，实现的难点就在于“如何将Instruction转换成Block”；其它部分，则相对容易一些。
那么，如何将Instruction转换成Block呢？我们可以依赖于Analyzer类的newControlFlowEdge方法来实现。
在Analyzer类中，newControlFlowEdge方法提供了一个空的实现：

protected void newControlFlowEdge(final int insnIndex, final int successorIndex) { // Nothing to do. }

如果我们仔细观察一下，就会发现：newControlFlowEdge方法是在analyze方法进行调用。
在analyze方法中，根据当前指令（insnNode）的类型，会对newControlFlowEdge方法的调用，有以下几种情况：

如果当前的insnNode是LabelNode、LineNumberNode或FrameNode类型
- 它们并不是真正意义上的instruction（不会存储到具体的.class文件中），只是起到辅助作用，按照顺序执行就可以了。
如果当前的insnNode是JumpInsnNode类型
- 它代表选择结构，判断是否要进行跳转。
- 注意，if语句有两个分支，一个是true，另一个是false；而goto就只有一个分支，不需要考虑true或false，直接跳转就可以了。
如果当前的insnNode是LookupSwitchInsnNode或TableSwitchInsnNode类型
- 它代表多重选择跳转，有多个分支结构。if语句只有两个分支，而switch可以支持更多分支，支持default和多个case的情况。
如果当前的insnNode是ATHROW或RETURN等代表结束的opcode，它表示方法的执行结束，没有任何的跳转分支。
其它情况（大多数的指令都是属于这种），按照顺序执行。

public class Analyzer< V extends Value> implements Opcodes { public Frame< V> [] analyze(final String owner, final MethodNode method) throws AnalyzerException { // Control flow analysis. while (numInstructionsToProcess > 0) { // Get and remove one instruction from the list of instructions to process. int insnIndex = instructionsToProcess[--numInstructionsToProcess]; // Simulate the execution of this instruction. AbstractInsnNode insnNode = null; try { insnNode = method.instructions.get(insnIndex); int insnOpcode = insnNode.getOpcode(); int insnType = insnNode.getType(); if (insnType == AbstractInsnNode.LABEL || insnType == AbstractInsnNode.LINE || insnType == AbstractInsnNode.FRAME) { newControlFlowEdge(insnIndex, insnIndex + 1); // 这里调用了newControlFlowEdge方法 } else { if (insnNode instanceof JumpInsnNode) { JumpInsnNode jumpInsn = (JumpInsnNode) insnNode; if (insnOpcode != GOTO & & insnOpcode != JSR) { newControlFlowEdge(insnIndex, insnIndex + 1); // 这里调用了newControlFlowEdge方法 } int jumpInsnIndex = insnList.indexOf(jumpInsn.label); newControlFlowEdge(insnIndex, jumpInsnIndex); // 这里调用了newControlFlowEdge方法 } else if (insnNode instanceof LookupSwitchInsnNode) { LookupSwitchInsnNode lookupSwitchInsn = (LookupSwitchInsnNode) insnNode; int targetInsnIndex = insnList.indexOf(lookupSwitchInsn.dflt); newControlFlowEdge(insnIndex, targetInsnIndex); // 这里调用了newControlFlowEdge方法 for (int i = 0; i < lookupSwitchInsn.labels.size(); ++i) { LabelNode label = lookupSwitchInsn.labels.get(i); targetInsnIndex = insnList.indexOf(label); newControlFlowEdge(insnIndex, targetInsnIndex); // 这里调用了newControlFlowEdge方法 } } else if (insnNode instanceof TableSwitchInsnNode) { TableSwitchInsnNode tableSwitchInsn = (TableSwitchInsnNode) insnNode; int targetInsnIndex = insnList.indexOf(tableSwitchInsn.dflt); newControlFlowEdge(insnIndex, targetInsnIndex); // 这里调用了newControlFlowEdge方法 for (int i = 0; i < tableSwitchInsn.labels.size(); ++i) { LabelNode label = tableSwitchInsn.labels.get(i); targetInsnIndex = insnList.indexOf(label); newControlFlowEdge(insnIndex, targetInsnIndex); // 这里调用了newControlFlowEdge方法 } } else if (insnOpcode != ATHROW & & (insnOpcode < IRETURN || insnOpcode > RETURN)) { newControlFlowEdge(insnIndex, insnIndex + 1); // 这里调用了newControlFlowEdge方法 } } } catch (AnalyzerException e) { throw new AnalyzerException(e.node, "Error at instruction " + insnIndex + ": " + e.getMessage(), e); } catch (RuntimeException e) { // DontCheck(IllegalCatch): cant be fixed, for backward compatibility. throw new AnalyzerException(insnNode, "Error at instruction " + insnIndex + ": " + e.getMessage(), e); } }return frames; } }

1.3. 图形部分
在项目当中提供了TextGraph类，可以用文本的形式将control flow graph打印出来。

public class HelloWorld { public void test(int val) { if (val == 0) { System.out.println("val is 0"); } else { System.out.println("val is unknown"); } } }

┌───────────────────────────────────┐ │ iload_1│ │ ifne L0├───┐ └────────────────┬──────────────────┘│ ││ ┌────────────────┴──────────────────┐│ │ getstatic System.out││ │ ldc "val is 0"││ │ invokevirtual PrintStream.println ││ │ goto L1├───┼──┐ └───────────────────────────────────┘││ ││ ┌───────────────────────────────────┐││ │ L0├───┘│ │ getstatic System.out││ │ ldc "val is unknown"││ │ invokevirtual PrintStream.println ││ └────────────────┬──────────────────┘│ ││ ┌────────────────┴──────────────────┐│ │ L1├──────┘ │ return│ └───────────────────────────────────┘

2. 代码实现接下来，我们要将抽象思路转换成具体的代码实现。

概念	类
instruction	`AbstractInsnNode`
block	`InsnBlock`
box	`InsnBox`
graph	`InsnGraph`

2.1. InsnBlock

import java.util.ArrayList; import java.util.List; public class InsnBlock { // 文字信息 public final List< String> lines = new ArrayList< > (); // 关联关系 public final List< InsnBlock> nextBlockList = new ArrayList< > (); public final List< InsnBlock> jumpBlockList = new ArrayList< > (); public void addLines(List< String> list) { lines.addAll(list); }public void addNext(InsnBlock item) { nextBlockList.add(item); }public void addJump(InsnBlock item) { jumpBlockList.add(item); }}

2.2. InsnBox

import lsieun.graphics.Rectangle; import lsieun.graphics.Text; public class InsnBox { private static final int INNER_PADDING = 10; private static final int RECTANGLE_WIDTH = 250; // 位置信息 public int x; public int y; // 图形信息 public Rectangle rectangle; public final InsnBlock block; public InsnBox(InsnBlock block) { this.block = block; }public void draw(int x, int y) { this.x = x; this.y = y; int currentX = x + INNER_PADDING; int currentY = y + INNER_PADDING; int currentWidth = 0; int currentHeight = 0; for (String item : block.lines) { Text text = new Text(currentX, currentY, item); text.draw(); int textWidth = text.getWidth() + 2 * INNER_PADDING; if (textWidth > currentWidth) { currentWidth = textWidth; } currentY += text.getHeight(); currentHeight = currentY - y; }int width = RECTANGLE_WIDTH; int height = currentHeight + INNER_PADDING; rectangle = new Rectangle(x, y, width, height); rectangle.draw(); }public int getWidth() { if (rectangle != null) { return rectangle.getWidth(); } else { return 0; } }public int getHeight() { if (rectangle != null) { return rectangle.getHeight(); } else { return 0; } } }

2.3. InsnGraph

public class InsnGraph { // 部分内容省略 private static final int START_X = 10; private static final int START_Y = 10; private final int startX; private final int startY; private final InsnBox[] boxes; public InsnGraph(InsnBlock[] blocks) { this(START_X, START_Y, blocks); }public InsnGraph(int startX, int startY, InsnBlock[] blocks) { this.startX = startX; this.startY = startY; int length = blocks.length; this.boxes = new InsnBox[length]; for (int i = 0; i < length; i++) { this.boxes[i] = new InsnBox(blocks[i]); } }public void draw() { int length = boxes.length; if (length < 1) return; drawBlockRectangles(); drawConnectionLines(); }private void drawBlockRectangles() { int currentX = this.startX; int currentY = this.startY; int length = boxes.length; for (int i = 0; i < length; i++) { InsnBox box = boxes[i]; if (i != 0) { InsnBox previousBox = boxes[i - 1]; currentY = previousBox.y + previousBox.getHeight() + ROW_SPACE; }box.draw(currentX, currentY); } }private void drawConnectionLines() { int length = boxes.length; for (int i = 0; i < length; i++) { InsnBox currentBox = boxes[i]; List< InsnBox> nextBoxList = findNextBoxes(currentBox); for (InsnBox nextBox : nextBoxList) { connectTop2BottomBlock(currentBox.rectangle, nextBox.rectangle); }List< InsnBox> jumpBoxList = findJumpBoxes(currentBox); for (InsnBox jumpbox : jumpBoxList) { jumpOne2Another(currentBox.rectangle, jumpbox.rectangle, i); } } }// 后续内容省略 }

2.4. 第一个版本
首先，我们先做一个简单的实现：每个Instruction生成一个Block。

import lsieun.asm.analysis.graph.InsnBlock; import org.objectweb.asm.tree.AbstractInsnNode; import org.objectweb.asm.tree.MethodNode; import org.objectweb.asm.tree.analysis.*; import java.util.List; public class ControlFlowEdgeAnalyzer< V extends Value> extends Analyzer< V> { private AbstractInsnNode[] nodeArray; public InsnBlock[] blocks; public ControlFlowEdgeAnalyzer(Interpreter< V> interpreter) { super(interpreter); }@Override public Frame< V> [] analyze(String owner, MethodNode method) throws AnalyzerException { nodeArray = method.instructions.toArray(); int length = nodeArray.length; blocks = new InsnBlock[length]; InsnText insnText = new InsnText(); for (int i = 0; i < length; i++) { blocks[i] = getBlock(i); AbstractInsnNode node = nodeArray[i]; List< String> lines = insnText.toLines(node); blocks[i].addLines(lines); }return super.analyze(owner, method); }@Override protected void newControlFlowEdge(int insnIndex, int successorIndex) { // 首先，处理自己的代码逻辑 AbstractInsnNode insnNode = nodeArray[insnIndex]; int insnOpcode = insnNode.getOpcode(); int insnType = insnNode.getType(); if (insnType == AbstractInsnNode.JUMP_INSN) { if ((insnIndex + 1) == successorIndex) { addNext(insnIndex, successorIndex); } else { addJump(insnIndex, successorIndex); } } else if (insnOpcode == LOOKUPSWITCH) { addJump(insnIndex, successorIndex); } else if (insnOpcode == TABLESWITCH) { addJump(insnIndex, successorIndex); } else if (insnOpcode == RET) { addJump(insnIndex, successorIndex); } else if (insnOpcode == ATHROW || (insnOpcode > = IRETURN & & insnOpcode < = RETURN)) { assert false : "should not be here"; removeNextAndJump(insnIndex); } else { addNext(insnIndex, successorIndex); }// 其次，调用父类的方法实现 super.newControlFlowEdge(insnIndex, successorIndex); }private void addNext(int fromIndex, int toIndex) { InsnBlock currentBlock = getBlock(fromIndex); InsnBlock nextBlock = getBlock(toIndex); currentBlock.addNext(nextBlock); }private void addJump(int fromIndex, int toIndex) { InsnBlock currentBlock = getBlock(fromIndex); InsnBlock nextBlock = getBlock(toIndex); currentBlock.addJump(nextBlock); }private void removeNextAndJump(int insnIndex) { InsnBlock currentBlock = getBlock(insnIndex); currentBlock.nextBlockList.clear(); currentBlock.jumpBlockList.clear(); }private InsnBlock getBlock(int insnIndex) { InsnBlock block = blocks[insnIndex]; if (block == null){ block = new InsnBlock(); blocks[insnIndex] = block; } return block; }public InsnBlock[] getBlocks() { return blocks; }}

2.5. 第二个版本
接着，我们在ControlFlowEdgeAnalyzer实现的基础上进一步扩展：将顺序执行多个Block合并为一个Block。

import lsieun.asm.analysis.graph.InsnBlock; import org.objectweb.asm.tree.analysis.Interpreter; import org.objectweb.asm.tree.analysis.Value; import java.util.ArrayList; import java.util.HashSet; import java.util.List; import java.util.Set; public class ControlFlowEdgeAnalyzer2< V extends Value> extends ControlFlowEdgeAnalyzer< V> {public ControlFlowEdgeAnalyzer2(Interpreter< V> interpreter) { super(interpreter); }@Override public InsnBlock[] getBlocks() { //（1）调用父类实现 InsnBlock[] blocks = super.getBlocks(); //（2）如果结果为空，则提前返回 if (blocks == null || blocks.length < 1) { return blocks; }//（3）记录“分隔点” Set< InsnBlock> newBlockSet = new HashSet< > (); int length = blocks.length; for (int i = 0; i < length; i++) { InsnBlock currentBlock = blocks[i]; List< InsnBlock> nextBlockList = currentBlock.nextBlockList; List< InsnBlock> jumpBlockList = currentBlock.jumpBlockList; boolean hasNext = false; boolean hasJump = false; if (nextBlockList.size() > 0) { hasNext = true; }if (jumpBlockList.size() > 0) { hasJump = true; }if (!hasNext & & (i + 1) < length) { newBlockSet.add(blocks[i + 1]); }if (hasJump) { newBlockSet.addAll(jumpBlockList); if (hasNext) { newBlockSet.add(blocks[i + 1]); } } }//（4）利用“分隔点”，合并成不同的分组 List< InsnBlock> resultList = new ArrayList< > (); for (int i = 0; i < length; i++) { InsnBlock currentBlock = blocks[i]; if (i == 0) { resultList.add(currentBlock); } else if (newBlockSet.contains(currentBlock)) { resultList.add(currentBlock); } else { int size = resultList.size(); InsnBlock lastBlock = resultList.get(size - 1); lastBlock.lines.addAll(currentBlock.lines); lastBlock.jumpBlockList.clear(); lastBlock.jumpBlockList.addAll(currentBlock.jumpBlockList); lastBlock.nextBlockList.clear(); lastBlock.nextBlockList.addAll(currentBlock.nextBlockList); } }return resultList.toArray(new InsnBlock[0]); } }

2.6. 验证结果

public class ControlFlowGraphRun { public static void main(String[] args) throws Exception { String relative_path = "sample/HelloWorld.class"; String filepath = FileUtils.getFilePath(relative_path); byte[] bytes = FileUtils.readBytes(filepath); //（1）构建ClassReader ClassReader cr = new ClassReader(bytes); //（2）生成ClassNode ClassNode cn = new ClassNode(); int parsingOptions = ClassReader.SKIP_DEBUG | ClassReader.SKIP_FRAMES; cr.accept(cn, parsingOptions); //（3）查找方法 String methodName = "test"; MethodNode targetNode = null; for (MethodNode mn : cn.methods) { if (mn.name.equals(methodName)) { targetNode = mn; break; } } if (targetNode == null) { System.out.println("Can not find method: " + methodName); return; }//（4）进行图形化显示 display(cn.name, targetNode, 2); //（5）打印复杂度 CyclomaticComplexity cc = new CyclomaticComplexity(); int complexity = cc.getCyclomaticComplexity(cn.name, targetNode); String line = String.format("%s:%s complexity: %d", targetNode.name, targetNode.desc, complexity); System.out.println(line); } }

3. 总结本文内容总结如下：

第一点，介绍生成control flow graph的思路：instruction --> block --> box --> graph。
第二点，在具体代码实现上，主要是依赖于Analyzer类的newControlFlowEdge方法。
- 第一个版本中，每一条instruction都生成一个block。
- 第二个版本中，将多个顺序执行的block合并成一个新的block。