我正在尝试为 an issue about missing stackmap frames 的解决方法编写单元测试，但为此目的，我需要生成一个类，如果缺少堆栈图帧，该类将无法在 Java 8 上进行验证。

下面你可以看到我的测试用例(依赖项:ASM、Guava、JUnit)。它从 GuineaPig 类中删除堆栈映射帧，希望导致其字节码无法验证。我遇到问题的部分是在 GuineaPig 中使用需要堆栈映射帧的最少代码填充 TODO，以便测试通过。

import com.google.common.io.*;
import org.junit.*;
import org.junit.rules.ExpectedException;
import org.objectweb.asm.*;

import java.io.*;

import static org.objectweb.asm.Opcodes.ASM5;

public class Java6MissingStackMapFrameFixerTest {

    @Rule
    public final ExpectedException thrown = ExpectedException.none();

    public static class GuineaPig {
        public GuineaPig() {
            // TODO: make me require stackmap frames
        }
    }

    @Test
    public void example_class_cannot_be_loaded_because_of_missing_stackmap_frame() throws Exception {
        byte[] originalBytecode = getBytecode(GuineaPig.class);

        ClassWriter cw = new ClassWriter(0);
        ClassVisitor cv = new ClassVisitor(ASM5, cw) {
            @Override
            public MethodVisitor visitMethod(int access, String name, String desc, String signature, String[] exceptions) {
                return new MethodVisitor(ASM5, super.visitMethod(access, name, desc, signature, exceptions)) {
                    @Override
                    public void visitFrame(int type, int nLocal, Object[] local, int nStack, Object[] stack) {
                        // remove the stackmap frames in order to cause a VerifyError
//                        super.visitFrame(type, nLocal, local, nStack, stack);
                    }

                };
            }
        };
        new ClassReader(originalBytecode).accept(cv, 0);

        byte[] transformedBytecode = cw.toByteArray();
//        Files.asByteSink(new File("test.class")).write(transformedBytecode);

        thrown.expect(VerifyError.class);
        thrown.expectMessage("Expecting a stackmap frame");
        Class<?> clazz = new TestingClassLoader().defineClass(transformedBytecode);
        clazz.newInstance();
    }

    private static byte[] getBytecode(Class<?> clazz) throws IOException {
        String classFile = clazz.getName().replace(".", "/") + ".class";
        try (InputStream b = clazz.getClassLoader().getResourceAsStream(classFile)) {
            return ByteStreams.toByteArray(b);
        }
    }

    private static class TestingClassLoader extends ClassLoader {

        public Class<?> defineClass(byte[] bytecode) {
            ClassReader cr = new ClassReader(bytecode);
            String className = cr.getClassName().replace("/", ".");
            return this.defineClass(className, bytecode, 0, bytecode.length);
        }
    }
}

最佳答案

理论

Java 虚拟机规范 §4.10.1 (通过类型检查验证)指定何时需要堆栈映射框架。首先它给出了一个非正式的描述:

The intent is that a stack map frame must appear at the beginning of each basic block in a method. The stack map frame specifies the verification type of each operand stack entry and of each local variable at the start of each basic block.

详细规范见§4.10.1.6 (带代码的类型检查方法)。 goto 命令需要堆栈映射框架:

It is illegal to have code after an unconditional branch without a stack map frame being provided for it.

和所有其他分支命令:

Branching to a target is type safe if the target has an associated stack frame, Frame, and the current stack frame, StackFrame, is assignable to Frame.

异常处理程序的开头也需要一个堆栈映射框架:

An instruction satisfies an exception handler if the instructions's outgoing type state is ExcStackFrame, and the handler's target (the initial instruction of the handler code) is type safe assuming an incoming type state T.

最后，§4.10.1.9 (类型检查指令)指定哪些指令需要带有堆栈映射框架的分支目标。在类型规则中寻找targetIsTypeSafe；指令 goto、if*、lookupswitch 和 tableswitch 都有它。

例子

即使是以下代码也需要堆栈映射帧:

public static class GuineaPig {
    public GuineaPig() {
        int i = 1;
        if (i > 0) {
            // code branch to require stackmap frames
        }
    }
}

如果缺少它们，代码将失败并出现异常:

java.lang.VerifyError: Expecting a stackmap frame at branch target 10
Exception Details:
  Location:
    net/orfjackal/retrolambda/Java6MissingStackMapFrameFixerTest$GuineaPig.<init>()V @7: ifle
  Reason:
    Expected stackmap frame at this location.
  Bytecode:
    0000000: 2ab7 000c 043c 1b9e 0003 b1            

        at java.lang.Class.getDeclaredConstructors0(Native Method)
        at java.lang.Class.privateGetDeclaredConstructors(Class.java:2658)
        at java.lang.Class.getConstructor0(Class.java:2964)
        at java.lang.Class.newInstance(Class.java:403)         

这是字节码:

  public net.orfjackal.retrolambda.Java6MissingStackMapFrameFixerTest$GuineaPig();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=2, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: iconst_1
         5: istore_1
         6: iload_1
         7: ifle          10
        10: return
      LineNumberTable:
        line 22: 0
        line 23: 4
        line 24: 6
        line 27: 10
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      11     0  this   Lnet/orfjackal/retrolambda/Java6MissingStackMapFrameFixerTest$GuineaPig;
            6       5     1     i   I
      StackMapTable: number_of_entries = 1
           frame_type = 255 /* full_frame */
          offset_delta = 10
          locals = [ class net/orfjackal/retrolambda/Java6MissingStackMapFrameFixerTest$GuineaPig, int ]
          stack = []

附言我花了一些时间才弄明白这一点，因为默认情况下，我使用代码覆盖率运行单元测试，而 IDEA 的代码覆盖率工具显然会自动重新计算所有类的堆栈图帧，这使我的测试删除它们的努力付诸东流。

关于java - 什么样的 Java 代码需要堆栈映射框架？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/24927993/