本文属于Java ASM系列三：Tree API当中的一篇。
1. Cyclomatic Complexity
Code complexity is important, yet difficult metric to measure.
One of the most relevant complexity measurement is the Cyclomatic Complexity(CC).
1.1. 直观视角
CC value can be calculated by measuring the number of independent execution paths of a program.
下面的HelloWorld类的test方法的复杂度是1。
public class HelloWorld { public void test() { System.out.println("Hello World"); }}其control flow graph如下：
┌───────────────────────────────────┐│ getstatic System.out ││ ldc "Hello World" ││ invokevirtual PrintStream.println ││ return │└───────────────────────────────────┘下面的HelloWorld类的test方法的复杂度是2。
public class HelloWorld { public void test(boolean flag) { if (flag) { System.out.println("flag is true"); } else { System.out.println("flag is false"); } }}其control flow graph如下：
┌───────────────────────────────────┐│ iload_1 ││ ifeq L0 ├───┐└─────────────────┬─────────────────┘ │ │ │┌─────────────────┴─────────────────┐ ││ getstatic System.out │ ││ ldc "flag is true" │ ││ invokevirtual PrintStream.println │ ││ goto L1 ├───┼──┐└───────────────────────────────────┘ │ │ │ │┌───────────────────────────────────┐ │ ││ L0 ├───┘ ││ getstatic System.out │ ││ ldc "flag is false" │ ││ invokevirtual PrintStream.println │ │└─────────────────┬─────────────────┘ │ │ │┌─────────────────┴─────────────────┐ ││ L1 ├──────┘│ return │└───────────────────────────────────┘下面的HelloWorld类的test方法的复杂度是3。
public class HelloWorld { public void test(boolean flag, boolean flag2) { if (flag && flag2) { System.out.println("both flags are true"); } else { System.out.println("one flag must be false"); } }}其control flow graph如下：
┌───────────────────────────────────┐│ iload_1 ││ ifeq L0 ├───┐└─────────────────┬─────────────────┘ │ │ │┌─────────────────┴─────────────────┐ ││ iload_2 │ ││ ifeq L0 ├───┼──┐└─────────────────┬─────────────────┘ │ │ │ │ │┌─────────────────┴─────────────────┐ │ ││ getstatic System.out │ │ ││ ldc "both flags are true" │ │ ││ invokevirtual PrintStream.println │ │ ││ goto L1 ├───┼──┼──┐└───────────────────────────────────┘ │ │ │ │ │ │┌───────────────────────────────────┐ │ │ ││ L0 ├───┴──┘ ││ getstatic System.out │ ││ ldc "one flag must be false" │ ││ invokevirtual PrintStream.println │ │└─────────────────┬─────────────────┘ │ │ │┌─────────────────┴─────────────────┐ ││ L1 ├─────────┘│ return │└───────────────────────────────────┘下面的HelloWorld类的test方法的复杂度是4。
public class HelloWorld { public void test(int value) { String result; switch (value) { case 10: result = "val = 1"; break; case 20: result = "val = 2"; break; case 30: result = "val = 3"; break; default: result = "val is unknown"; } System.out.println(result); }}其control flow graph如下：
┌───────────────────────────────────┐│ iload_1 ││ lookupswitch { ││ 10: L0 ││ 20: L1 ││ 30: L2 ││ default: L3 ││ } ├───┐└───────────────────────────────────┘ │ │┌───────────────────────────────────┐ ││ L0 ├───┤│ ldc "val = 1" │ ││ astore_2 │ ││ goto L4 ├───┼──┐└───────────────────────────────────┘ │ │ │ │┌───────────────────────────────────┐ │ ││ L1 ├───┤ ││ ldc "val = 2" │ │ ││ astore_2 │ │ ││ goto L4 ├───┼──┼──┐└───────────────────────────────────┘ │ │ │ │ │ │┌───────────────────────────────────┐ │ │ ││ L2 ├───┤ │ ││ ldc "val = 3" │ │ │ ││ astore_2 │ │ │ ││ goto L4 ├───┼──┼──┼──┐└───────────────────────────────────┘ │ │ │ │ │ │ │ │┌───────────────────────────────────┐ │ │ │ ││ L3 ├───┘ │ │ ││ ldc "val is unknown" │ │ │ ││ astore_2 │ │ │ │└─────────────────┬─────────────────┘ │ │ │ │ │ │ │┌─────────────────┴─────────────────┐ │ │ ││ L4 ├──────┴──┴──┘│ getstatic System.out ││ aload_2 ││ invokevirtual PrintStream.println ││ return │└───────────────────────────────────┘下面的HelloWorld类的test方法的复杂度是5。
public class HelloWorld { public void test(int i) { String result = i % 2 == 0 ? "a" : i % 3 == 0 ? "b" : i % 5 == 0 ? "c" : i % 7 == 0 ? "d" : "e"; System.out.println(result); }}其control flow graph如下：
┌───────────────────────────────────┐│ iload_1 ││ iconst_2 ││ irem ││ ifne L0 ├───┐└─────────────────┬─────────────────┘ │ │ │┌─────────────────┴─────────────────┐ ││ ldc "a" │ ││ goto L1 ├───┼──┐└───────────────────────────────────┘ │ │ │ │┌───────────────────────────────────┐ │ ││ L0 ├───┘ ││ iload_1 │ ││ iconst_3 │ ││ irem │ ││ ifne L2 ├──────┼──┐└─────────────────┬─────────────────┘ │ │ │ │ │┌─────────────────┴─────────────────┐ │ ││ ldc "b" │ │ ││ goto L1 ├──────┼──┼──┐└───────────────────────────────────┘ │ │ │ │ │ │┌───────────────────────────────────┐ │ │ ││ L2 ├──────┼──┘ ││ iload_1 │ │ ││ iconst_5 │ │ ││ irem │ │ ││ ifne L3 ├──────┼─────┼──┐└─────────────────┬─────────────────┘ │ │ │ │ │ │ │┌─────────────────┴─────────────────┐ │ │ ││ ldc "c" │ │ │ ││ goto L1 ├──────┼─────┼──┼──┐└───────────────────────────────────┘ │ │ │ │ │ │ │ │┌───────────────────────────────────┐ │ │ │ ││ L3 ├──────┼─────┼──┘ ││ iload_1 │ │ │ ││ bipush 7 │ │ │ ││ irem │ │ │ ││ ifne L4 ├──────┼─────┼─────┼──┐└─────────────────┬─────────────────┘ │ │ │ │ │ │ │ │ │┌─────────────────┴─────────────────┐ │ │ │ ││ ldc "d" │ │ │ │ ││ goto L1 ├──────┼─────┼─────┼──┼──┐└───────────────────────────────────┘ │ │ │ │ │ │ │ │ │ │┌───────────────────────────────────┐ │ │ │ │ ││ L4 ├──────┼─────┼─────┼──┘ ││ ldc "e" │ │ │ │ │└─────────────────┬─────────────────┘ │ │ │ │ │ │ │ │ │┌─────────────────┴─────────────────┐ │ │ │ ││ L1 ├──────┴─────┴─────┴─────┘│ astore_2 ││ getstatic System.out ││ aload_2 ││ invokevirtual PrintStream.println ││ return │└───────────────────────────────────┘1.2. 数学视角
The complexity M is then defined as
M = E - N + 2Pwhere
E= the number of edges of the graph.N= the number of nodes of the graph.P= the number of connected components.
For a single program, P is always equal to 1. So a simpler formula for a single subroutine is
M = E - N + 21.3. 复杂度分级
The complexity level affects the testability of the code, the higher the CC, the higher the difficulty to implement pertinent tests.
In fact, the cyclomatic complexity value shows exactly the number of test cases needed to achieve a 100% branches coverage score.
Some common values used by static analysis tools are shown below:
1-4: low complexity – easy to test5-7: moderate complexity – tolerable8-10: high complexity – refactoring should be considered to ease testing11+ very high complexity – very difficult to test
Generally speaking, a code with a value higher than 11 in terms of CC, is considered very complex, and difficult to test and maintain.
2. 示例：计算圈复杂度
2.1. 预期目标
假如有一个HelloWorld类，代码如下：
public class HelloWorld { public void test(int val) { if (val == 0) { System.out.println("val is 0"); } else if (val == 1) { System.out.println("val is 1"); } else { System.out.println("val is unknown"); } }}我们的预期目标：确定test方法的复杂度。
2.2. 编码实现
在下面的CyclomaticComplexityFrame类当中，关键点是定义了一个successors字段，用来记录当前Frame与其它Frame之间的关联关系。
import org.objectweb.asm.tree.analysis.Frame;import org.objectweb.asm.tree.analysis.Value;import java.util.HashSet;import java.util.Set;public class CyclomaticComplexityFrame extends Frame { public Set> successors = new HashSet<>(); public CyclomaticComplexityFrame(int numLocals, int numStack) { super(numLocals, numStack); } public CyclomaticComplexityFrame(Frame frame) { super(frame); }} 在下面的CyclomaticComplexityAnalyzer类当中，从两点来把握：
- 第一点，两个
newFrame方法是用来替换掉默认的Frame类，而使用上面定义的CyclomaticComplexityFrame类。 - 第二点，修改
newControlFlowEdge方法，记录Frame之间的关联关系。
import org.objectweb.asm.tree.analysis.*;public class CyclomaticComplexityAnalyzer extends Analyzer { public CyclomaticComplexityAnalyzer(Interpreter interpreter) { super(interpreter); } @Override protected Frame newFrame(int numLocals, int numStack) { return new CyclomaticComplexityFrame<>(numLocals, numStack); } @Override protected Frame newFrame(Frame frame) { return new CyclomaticComplexityFrame<>(frame); } @Override protected void newControlFlowEdge(int insnIndex, int successorIndex) { CyclomaticComplexityFrame frame = (CyclomaticComplexityFrame) getFrames()[insnIndex]; frame.successors.add((CyclomaticComplexityFrame) getFrames()[successorIndex]); }} 在下面的CyclomaticComplexity类当中，应用M = E - N + 2公式：
import org.objectweb.asm.tree.MethodNode;import org.objectweb.asm.tree.analysis.*;public class CyclomaticComplexity { public static int getCyclomaticComplexity(String owner, MethodNode mn) throws AnalyzerException { // 第一步，获取Frame信息 Analyzer analyzer = new CyclomaticComplexityAnalyzer<>(new BasicInterpreter()); Frame[] frames = analyzer.analyze(owner, mn); // 第二步，计算复杂度 int edges = 0; int nodes = 0; for (Frame frame : frames) { if (frame != null) { edges += ((CyclomaticComplexityFrame) frame).successors.size(); nodes += 1; } } return edges - nodes + 2; }} 2.3. 进行分析
public class HelloWorldAnalysisTree { 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 int api = Opcodes.ASM9; ClassNode cn = new ClassNode(api); int parsingOptions = ClassReader.SKIP_DEBUG | ClassReader.SKIP_FRAMES; cr.accept(cn, parsingOptions); //（3）进行分析 String className = cn.name; List methods = cn.methods; for (MethodNode mn : methods) { int complexity = CyclomaticComplexity.getCyclomaticComplexity(className, mn); String line = String.format("%s:%s%n complexity: %d", mn.name, mn.desc, complexity); System.out.println(line); } }} 2.4. 验证结果
:()V complexity: 1test:(I)V complexity: 3 另外，要说明一点，Cyclomatic Complexity计算的是return的复杂度。
下面的HelloWorld类的test方法的复杂度是2。
public class HelloWorld { public void test(int val) { if (val == 0) { System.out.println("val is 0"); } else if (val == 1) { throw new RuntimeException("val is 1"); // 注意，这里抛出异常 } else { System.out.println("val is unknown"); } }}下面的HelloWorld类的test方法的复杂度是1。
public class HelloWorld { public void test(int val) { if (val == 0) { System.out.println("val is 0"); } else if (val == 1) { throw new RuntimeException("val is 1"); // 注意，这里抛出异常 } else { throw new RuntimeException("val is unknown"); // 注意，这里抛出异常 } }}3. 总结
本文内容总结如下：
- 第一点，介绍Cyclomatic Complexity的概念，如何用数学公式表达，以及复杂度分级。
- 第二点，代码示例，如何使用Java ASM实现Cyclomatic Complexity计算。
