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coverage-counters.adoc

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覆蓋率計算方式 (Coverage Counters)

先從 instruction coverage 談起，前面多次提到 bytecode instruction，但為什麼 source code 有 10 行 (L63 - L72)，而 instruction coverage 卻顯示 product() 有 21 個 instruction 被覆蓋，還有 8 個 instruction 沒被覆蓋？

example coverage report method instructions

用 javap 反解 ExpressionParser class 的 bytecode：

$ javap -c -p target/classes/org/jacoco/examples/parser/ExpressionParser.class
Compiled from "ExpressionParser.java"
public class org.jacoco.examples.parser.ExpressionParser {
  ...
  private org.jacoco.examples.expressions.IExpression product() throws java.io.IOException;
    Code:
       0: aload_0
       1: invokespecial #17                 // Method factor:()Lorg/jacoco/examples/expressions/IExpression;
       4: astore_1
       5: aload_0
       6: bipush        42
       8: invokespecial #12                 // Method accept:(I)Z
      11: ifeq          30
      14: new           #18                 // class org/jacoco/examples/expressions/Mul
      17: dup
      18: aload_1
      19: aload_0
      20: invokespecial #17                 // Method factor:()Lorg/jacoco/examples/expressions/IExpression;
      23: invokespecial #19                 // Method org/jacoco/examples/expressions/Mul."<init>":(Lorg/jacoco/examples/expressions/IExpression;Lorg/jacoco/examples/expressions/IExpression;)V
      26: astore_1
      27: goto          5
      30: aload_0
      31: bipush        47
      33: invokespecial #12                 // Method accept:(I)Z
      36: ifeq          55
      39: new           #20                 // class org/jacoco/examples/expressions/Div
      42: dup
      43: aload_1
      44: aload_0
      45: invokespecial #17                 // Method factor:()Lorg/jacoco/examples/expressions/IExpression;
      48: invokespecial #21                 // Method org/jacoco/examples/expressions/Div."<init>":(Lorg/jacoco/examples/expressions/IExpression;Lorg/jacoco/examples/expressions/IExpression;)V
      51: astore_1
      52: goto          5
      55: aload_1
      56: areturn

  ...
}

算一算，從 0: aload_0 到 56: areturn 剛好就 21 + 8 = 29 個 bytecode instruction。

$ cat Main.java
public class Main {

    private static final String GREETING = new String("Hello, World!");

    public static void main(String[] args) {
        System.out.println(GREETING);
    }
}
$ javac Main.java
$ javap -c Main.class
Compiled from "Main.java"
public class Main {
  public Main();
    Code: (1)
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
       3: getstatic     #3                  // Field GREETING:Ljava/lang/String;
       6: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
       9: return

  static {};
    Code: (3)
       0: new           #5                  // class java/lang/String
       3: dup
       4: ldc           #6                  // String Hello, World!
       6: invokespecial #7                  // Method java/lang/String."<init>":(Ljava/lang/String;)V
       9: putstatic     #3                  // Field GREETING:Ljava/lang/String;
      12: return
}

  1. 自動產生的 default constructor，有 3 個 bytecode instruction。

  2. System.out.println(GREETING) 一行 Java code 產生 4 個 bytecode instruction。

  3. 因為要初始化常數 GREETING 的關係，自動產生了一個 static initializer。

主要以 bytecode 為主，不一定有對應的 Java source code。