Java 20解构特性疑似Bug：启用switch分支触发VerifyError

我写了一段使用模式匹配switch的Java代码，原本编译运行能得到预期输出，但保留switch语句中带when守卫的Point解构分支后，代码能正常编译，但在启用预览特性的MacOS 13.3.1 ARM64 JDK 20环境中运行时，抛出了java.lang.VerifyError错误。

测试代码 #

import static java.lang.System.*;
public class Main {
    record Point(int x, int y, Color color){}
    enum Color {RED, BLUE, YELLOW}
    public static void main(String[] args) {
        testSwitch(new Point(10,20,Color.BLUE));
        testSwitch(new Point(10,20,Color.RED));
        testSwitch("vbjhjbhbjhbj");
        testSwitch(1.012);
        testSwitch(Color.YELLOW);
    }
    static void testSwitch(Object o){
        switch(o){
            case null -> out.println("Null");
            case Color c -> out.println("Color: "+c);
            case Point(var x, var y, Color c) when c == Color.RED -> out.printf("Red point x=%d, y=%d\n",x,y);
            case Point(var x, var y, var color) -> out.printf("Point x=%d, y=%d, color=%s\n",x,y,color);
            case String s when s.length() >=7 -> out.printf("Big string: '%s', of size %d\n",s,s.length());
            case String s -> out.printf("Small string: %s, of size %d\n",s,s.length());
            default -> out.println("Unexpected value: " + o);
        }
    }
}

运行错误信息 #

/Library/Java/JavaVirtualMachines/jdk-20.jdk/Contents/Home/bin/java --enable-preview -javaagent:/Applications/IntelliJ IDEA.app/Contents/lib/idea_rt.jar=50788:/Applications/IntelliJ IDEA.app/Contents/bin -Dfile.encoding=UTF-8 -Dsun.stdout.encoding=UTF-8 -Dsun.stderr.encoding=UTF-8 -classpath /Users/hristo/projects/java20tests/java20tests/out/production/java20tests Main
Error: Unable to initialize main class Main
Caused by: java.lang.VerifyError: Bad local variable type
Exception Details:
  Location:
    Main.testSwitch(Ljava/lang/Object;)V @309: aload
  Reason:
    Type top (current frame, locals[7]) is not assignable to reference type
  Current Frame:
    bci: @309
    flags: { }
    locals: { 'java/lang/Object', 'java/lang/Object', integer, top, 'Main$Point', integer, integer, top, integer, integer, integer, integer, 'Main$Color', integer }
    stack: { 'java/io/PrintStream', 'java/lang/String', '[Ljava/lang/Object;', '[Ljava/lang/Object;', integer }
  Bytecode:
    0000000: 2a4c 033d 2b1c ba00 2800 00aa 0000 019a
    0000010: ffff ffff 0000 0003 0000 0021 0000 002c
    0000020: 0000 0043 0000 013f 0000 0174 b200 2c12
    0000030: 32b6 0034 a701 832b c000 0a4e b200 2c2d
    0000040: b800 3aba 003f 0000 b600 34a7 016c 2bc0
    0000050: 0007 3a04 1904 b600 4336 0803 3609 1508
    0000060: b800 4715 09ba 004c 0000 ab00 0000 00db
    0000070: 0000 0002 ffff ffff 0000 00db 0000 0000
    0000080: 0000 001a 1508 3605 1904 b600 4d36 0a03
    0000090: 360b 150a b800 4715 0bba 004c 0000 ab00
    00000a0: 0000 00a1 0000 0002 ffff ffff 0000 00a1
    00000b0: 0000 0000 0000 001a 150a 3606 1904 b600
    00000c0: 503a 0c03 360d 190c 150d ba00 5400 00ab
    00000d0: 0000 004b 0000 0002 ffff ffff 0000 004b
    00000e0: 0000 0000 0000 0019 190c 3a07 1907 b200
    00000f0: 18a5 0009 0436 0da7 ffcf b200 2c12 5805
    0000100: bd00 0259 0315 05b8 0047 5359 0415 06b8
    0000110: 0047 53b6 005a 57a7 00a0 b200 2c12 5e06
    0000120: bd00 0259 0315 05b8 0047 5359 0415 06b8
    0000130: 0047 5359 0519 0753 b600 5a57 a700 7b04
    0000140: 3609 a7ff 1c05 3da7 febd 2bc0 003b 3a08
    0000150: 1908 b600 6010 07a2 0008 063d a7fe a8b2
    0000160: 002c 1263 05bd 0002 5903 1908 5359 0419
    0000170: 08b6 0060 b800 4753 b600 5a57 a700 3b2b
    0000180: c000 3b3a 09b2 002c 1265 05bd 0002 5903
    0000190: 1909 5359 0419 09b6 0060 b800 4753 b600
    00001a0: 5a57 a700 15b2 002c 2ab8 003a ba00 6700
    00001b0: 00b6 0034 a700 03a7 0011 4cbb 006a 592b
    00001c0: b600 6c2b b700 70bf b1                 
  Exception Handler Table:
    bci [86, 89] => handler: 442
    bci [138, 141] => handler: 442
    bci [190, 193] => handler: 442
  Stackmap Table:
    append_frame(@4,Object[#2],Integer)
    same_frame(@44)
    same_frame(@55)
    same_frame(@78)
    full_frame(@94,{Object[#2],Object[#2],Integer,Top,Object[#7],Top,Top,Top,Integer,Integer},{})
    same_frame(@132)
    full_frame(@146,{Object[#2],Object[#2],Integer,Top,Object[#7],Integer,Top,Top,Integer,Integer,Integer,Integer},{})
    same_frame(@184)
    full_frame(@198,{Object[#2],Object[#2],Integer,Top,Object[#7],Integer,Integer,Top,Integer,Integer,Integer,Integer,Object[#10],Integer},{})
    same_frame(@232)
    full_frame(@250,{Object[#2],Object[#2],Integer,Top,Object[#7],Integer,Integer,Object[#10],Integer,Integer,Integer,Integer,Object[#10],Integer},{})
    full_frame(@282,{Object[#2],Object[#2],Integer,Top,Object[#7],Integer,Integer,Top,Integer,Integer,Integer,Integer,Object[#10],Integer},{})
    full_frame(@319,{Object[#2],Object[#2],Integer,Top,Object[#7],Integer,Top,Top,Integer,Integer,Integer,Integer},{})
    full_frame(@325,{Object[#2],Object[#2],Integer,Top,Object[#7],Top,Top,Top,Integer,Integer},{})
    full_frame(@330,{Object[#2],Object[#2],Integer},{})
    full_frame(@351,{Object[#2],Object[#2],Integer,Top,Top,Top,Top,Top,Object[#59]},{})
    full_frame(@383,{Object[#2],Object[#2],Integer},{})
    same_frame(@421)
    chop_frame(@439,2)
    same_locals_1_stack_item_frame(@442,Object[#104])
    same_frame(@456)

Process finished with exit code 1

结论与解决建议 #

这是Java 20的Bug，和你的代码逻辑、理解无关。具体是JDK编译器在处理带when守卫的record解构模式匹配switch分支时，生成的字节码不符合JVM的验证规则，导致运行时触发VerifyError。这类问题属于预览特性的实现缺陷，因为模式匹配switch在Java 20中还处于预览阶段，正式版的Java 21已经修复了大量此类问题。

解决建议：

• 直接升级到Java 21，正式版的模式匹配switch已经稳定，不会出现这类验证错误
• 如果必须使用Java 20，可以尝试调整分支顺序（比如把带when的Point分支放到普通Point分支之后），或者换一种写法避开解构+when的组合
• 等待Oracle发布JDK 20的补丁修复该问题

内容的提问来源于stack exchange，提问作者Hristo Stoyanov