字节码层面
通过javac Singleton.java将类编译为class文件，再通过javap -v -p Singleton.class命令反编译查看字节码文件。-p的作用是显示所有类与成员
D:\JavaSE\JavaProject\design-pattern\src\main\java>javap -p -v Singleton.class
Classfile /D:/JavaSE/JavaProject/design-pattern/src/main/java/Singleton.class
Last modified 2021-4-19; size 509 bytes
MD5 checksum fc6fcd094d2d9cdf0edd20d59c6b0d22
Compiled from "Singleton.java"
public class Singleton
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #5.#20 // java/lang/Object."<init>":()V
#2 = Fieldref #3.#21 // Singleton.singleton:LSingleton;
#3 = Class #22 // Singleton
#4 = Methodref #3.#20 // Singleton."<init>":()V
#5 = Class #23 // java/lang/Object
#6 = Utf8 singleton
#7 = Utf8 LSingleton;
#8 = Utf8 <init>
#9 = Utf8 ()V
#10 = Utf8 Code
#11 = Utf8 LineNumberTable
#12 = Utf8 getInstance
#13 = Utf8 ()LSingleton;
#14 = Utf8 StackMapTable
#15 = Class #23 // java/lang/Object
#16 = Class #24 // java/lang/Throwable
#17 = Utf8 <clinit>
#18 = Utf8 SourceFile
#19 = Utf8 Singleton.java
#20 = NameAndType #8:#9 // "<init>":()V
#21 = NameAndType #6:#7 // singleton:LSingleton;
#22 = Utf8 Singleton
#23 = Utf8 java/lang/Object
#24 = Utf8 java/lang/Throwable
{
private static volatile Singleton singleton;

descriptor: LSingleton;flags: [Skrill下载](https://www.gendan5.com/wallet/Skrill.html)ACC_PRIVATE, ACC_STATIC, ACC_VOLATILE

private Singleton();

descriptor: ()Vflags: ACC_PRIVATECode:  stack=1, locals=1, args_size=1     0: aload_0     1: invokespecial #1                  // Method java/lang/Object."<init>":()V     4: return  LineNumberTable:    line 3: 0    line 5: 4

public static Singleton getInstance();

descriptor: ()LSingleton;flags: ACC_PUBLIC, ACC_STATICCode:  stack=2, locals=2, args_size=0     0: getstatic     #2                  // Field singleton:LSingleton;     3: ifnonnull     37     6: ldc           #3                  // class Singleton     8: dup     9: astore_0    10: monitorenter    11: getstatic     #2                  // Field singleton:LSingleton;    14: ifnonnull     27    17: new           #3                  // class Singleton    20: dup    21: invokespecial #4                  // Method "<init>":()V    24: putstatic     #2                  // Field singleton:LSingleton;    27: aload_0    28: monitorexit    29: goto          37    32: astore_1    33: aload_0    34: monitorexit    35: aload_1    36: athrow    37: getstatic     #2                  // Field singleton:LSingleton;    40: areturn  Exception table:     from    to  target type        11    29    32   any        32    35    32   any  LineNumberTable:    line 7: 0    line 8: 6    line 9: 11    line 10: 17    line 12: 27    line 14: 37  StackMapTable: number_of_entries = 3    frame_type = 252 /* append */      offset_delta = 27      locals = [ class java/lang/Object ]    frame_type = 68 /* same_locals_1_stack_item */      stack = [ class java/lang/Throwable ]    frame_type = 250 /* chop */      offset_delta = 4

static {};

descriptor: ()Vflags: ACC_STATICCode:  stack=1, locals=0, args_size=0     0: aconst_null     1: putstatic     #2                  // Field singleton:LSingleton;     4: return  LineNumberTable:    line 2: 0

}
SourceFile: "Singleton.java"