Synthetic是人造，合成的意思，在虚拟机很多地方使用ACC_SYNTHETIC表示编译器自动生成的，区别于我们自己写的程序代码。这样说可能比较模糊，我们举个例子：我们创建一个内部类，如下
public class TestInnerClass {public void say(String msg) {System.out.println(msg);}class InnerClass {public void sayInner() {say("hello");}}
}我们用javap看一下内部类InnerClass反编译之后的情况：
完整如下：
Classfile /F:/demo/mallsystem/provider/target/classes/com/mall/test/TestInnerClass$InnerClass.classLast modified 2024-11-28; size 638 bytesMD5 checksum 9f5efd7b8d0ad50545773d2a0e198f60Compiled from "TestInnerClass.java"
class com.mall.test.TestInnerClass$InnerClassminor version: 0major version: 52flags: ACC_SUPER
Constant pool:#1 = Fieldref #5.#22 // com/mall/test/TestInnerClass$InnerClass.this$0:Lcom/mall/test/TestInnerClass;#2 = Methodref #6.#23 // java/lang/Object."<init>":()V#3 = String #24 // hello#4 = Methodref #25.#26 // com/mall/test/TestInnerClass.say:(Ljava/lang/String;)V#5 = Class #27 // com/mall/test/TestInnerClass$InnerClass#6 = Class #28 // java/lang/Object#7 = Utf8 this$0#8 = Utf8 Lcom/mall/test/TestInnerClass;#9 = Utf8 <init>#10 = Utf8 (Lcom/mall/test/TestInnerClass;)V#11 = Utf8 Code#12 = Utf8 LineNumberTable#13 = Utf8 LocalVariableTable#14 = Utf8 this#15 = Utf8 InnerClass#16 = Utf8 InnerClasses#17 = Utf8 Lcom/mall/test/TestInnerClass$InnerClass;#18 = Utf8 sayInner#19 = Utf8 ()V#20 = Utf8 SourceFile#21 = Utf8 TestInnerClass.java#22 = NameAndType #7:#8 // this$0:Lcom/mall/test/TestInnerClass;#23 = NameAndType #9:#19 // "<init>":()V#24 = Utf8 hello#25 = Class #29 // com/mall/test/TestInnerClass#26 = NameAndType #30:#31 // say:(Ljava/lang/String;)V#27 = Utf8 com/mall/test/TestInnerClass$InnerClass#28 = Utf8 java/lang/Object#29 = Utf8 com/mall/test/TestInnerClass#30 = Utf8 say#31 = Utf8 (Ljava/lang/String;)V
{final com.mall.test.TestInnerClass this$0;descriptor: Lcom/mall/test/TestInnerClass;flags: ACC_FINAL, ACC_SYNTHETICcom.mall.test.TestInnerClass$InnerClass(com.mall.test.TestInnerClass);descriptor: (Lcom/mall/test/TestInnerClass;)Vflags:Code:stack=2, locals=2, args_size=20: aload_01: aload_12: putfield #1 // Field this$0:Lcom/mall/test/TestInnerClass;5: aload_06: invokespecial #2 // Method java/lang/Object."<init>":()V9: returnLineNumberTable:line 12: 0LocalVariableTable:Start Length Slot Name Signature0 10 0 this Lcom/mall/test/TestInnerClass$InnerClass;0 10 1 this$0 Lcom/mall/test/TestInnerClass;public void sayInner();descriptor: ()Vflags: ACC_PUBLICCode:stack=2, locals=1, args_size=10: aload_01: getfield #1 // Field this$0:Lcom/mall/test/TestInnerClass;4: ldc #3 // String hello6: invokevirtual #4 // Method com/mall/test/TestInnerClass.say:(Ljava/lang/String;)V9: returnLineNumberTable:line 14: 0line 15: 9LocalVariableTable:Start Length Slot Name Signature0 10 0 this Lcom/mall/test/TestInnerClass$InnerClass;
}
SourceFile: "TestInnerClass.java"
InnerClasses:#15= #5 of #25; //InnerClass=class com/mall/test/TestInnerClass$InnerClass of class com/mall/test/TestInnerClass重点看下面标记的，ACC_SYNTHETIC标记出现了
这里编译器在内部类生成了一个TestInnerClass的引用对象this$0，因为是编译器自动合成的，所以标记了ACC_SYNTHETIC。
而下面是内部类的构造方法，可以看到它传入了外部类的实例对象，之后把它赋值给this$0
aload_0默认是实例方法第一个参数是this，即内部类自己的引用
aload_1则是加载第一个入参也就是外部类实例对象
putfield #1就是把外部实例设置给this$0
所以这也就解释了为什么内部类创建需要外部类对象创建了，如果外部类对象还没有生成，怎么传给内部类的构造方法呢？
而我们内部类能调用外部类的方法，也正是通过外部类的实例对象调用的