我们在阅读JDK反射包源码时，会遇到isSynthetic()方法，其实现之一是Modifier.isSynthetic(getModifiers())，其他方式的原理一样。Modifier是专门定义修饰符的类，其中static final int SYNTHETIC = 0x00001000 表名synthetic是修饰符的一种。
我尝试用synthetic来修饰自定义的Class/Field/Method/Constructer。但是都会编译失败，理由是不认识synthetic修饰符。那么synthetic到底是怎么使用的呢？
官方定义
我从Java语言规范中找到了定义：
Any constructs introduced by the compiler that do not have a corresponding construct in the source code must be marked as synthetic ,except for default constructs and the class initialization method.
意思大概是这样：任何被编译器引入的构造器在源代码中没有一个相应的构造器，那么该段代码就必须要被标记为synthetic的(复合的)，除了默认的构造器和类初始化方法。
验证
基于上面的定义，我们知道synthetic是由编译器生成的，而且synthetic有复合的、合成的意思。所以猜测是否是用在内部类中的。我们先构造一个内部类：
public class SyntheticDemo {
public static void main(String[] args) {
InnerClass innerObject = new InnerClass();
System.out.println("inner: " + innerObject.inner);
}
private static class InnerClass{
private String inner = "我在内部";
}
}编译后生成了三个文件：
- SyntheticDemo$1.class
- SyntheticDemo$InnerClass.class
- SyntheticDemo.class
我们依次反编译一下这三个class文件
SyntheticDemo$1.class
javap SyntheticDemo$1.class:
Compiled from "SyntheticDemo.java"
class compiler.SyntheticDemo$1 {
}
javap -v SyntheticDemo$1.class
Last modified 2019-12-12; size 205 bytes
MD5 checksum 942560dec8fdbc3d9b65e31e4b41295a
Compiled from "SyntheticDemo.java"
class compiler.SyntheticDemo$1
minor version: 0
major version: 52
flags: ACC_SUPER, ACC_SYNTHETIC
Constant pool:
#1 = Class #7 // compiler/SyntheticDemo$1
#2 = Class #9 // java/lang/Object
#3 = Utf8 SourceFile
#4 = Utf8 SyntheticDemo.java
#5 = Utf8 EnclosingMethod
#6 = Class #10 // compiler/SyntheticDemo
#7 = Utf8 compiler/SyntheticDemo$1
#8 = Utf8 InnerClasses
#9 = Utf8 java/lang/Object
#10 = Utf8 compiler/SyntheticDemo
{
}
SourceFile: "SyntheticDemo.java"
EnclosingMethod: #6.#0 // compiler.SyntheticDemo
InnerClasses:
static #1; //class compiler/SyntheticDemo$1
这个类里面什么都没有，并且此类的flags包含ACC_SYNTHETIC。
SyntheticDemo$InnerClass.class
javap SyntheticDemo$InnerClass.class：
Compiled from "SyntheticDemo.java"
class compiler.SyntheticDemo$InnerClass {
compiler.SyntheticDemo$InnerClass(compiler.SyntheticDemo$1);
static java.lang.String access$100(compiler.SyntheticDemo$InnerClass);
}
javap SyntheticDemo$InnerClass.class：
Classfile /D:/idea_workspace/mycode/target/classes/compiler/SyntheticDemo$InnerClass.class
Last modified 2019-12-12; size 763 bytes
MD5 checksum f13a4310236918575d26a2909aa8507e
Compiled from "SyntheticDemo.java"
class compiler.SyntheticDemo$InnerClass
minor version: 0
major version: 52
flags: ACC_SUPER
Constant pool:
#1 = Fieldref #5.#26 // compiler/SyntheticDemo$InnerClass.inner:Ljava/lang/String;
#2 = Methodref #5.#27 // compiler/SyntheticDemo$InnerClass."":()V
#3 = Methodref #6.#27 // java/lang/Object."":()V
#4 = String #28 // 我在内部
#5 = Class #30 // compiler/SyntheticDemo$InnerClass
#6 = Class #31 // java/lang/Object
#7 = Utf8 inner
#8 = Utf8 Ljava/lang/String;
#9 = Utf8
#10 = Utf8 ()V
#11 = Utf8 Code
#12 = Utf8 LineNumberTable
#13 = Utf8 LocalVariableTable
#14 = Utf8 this
#15 = Utf8 InnerClass
#16 = Utf8 InnerClasses
#17 = Utf8 Lcompiler/SyntheticDemo$InnerClass;
#18 = Class #32 // compiler/SyntheticDemo$1
#19 = Utf8 (Lcompiler/SyntheticDemo$1;)V
#20 = Utf8 x0
#21 = Utf8 Lcompiler/SyntheticDemo$1;
#22 = Utf8 access$100
#23 = Utf8 (Lcompiler/SyntheticDemo$InnerClass;)Ljava/lang/String;
#24 = Utf8 SourceFile
#25 = Utf8 SyntheticDemo.java
#26 = NameAndType #7:#8 // inner:Ljava/lang/String;
#27 = NameAndType #9:#10 // "":()V
#28 = Utf8 我在内部
#29 = Class #33 // compiler/SyntheticDemo
#30 = Utf8 compiler/SyntheticDemo$InnerClass
#31 = Utf8 java/lang/Object
#32 = Utf8 compiler/SyntheticDemo$1
#33 = Utf8 compiler/SyntheticDemo
{
compiler.SyntheticDemo$InnerClass(compiler.SyntheticDemo$1);
descriptor: (Lcompiler/SyntheticDemo$1;)V
flags: ACC_SYNTHETIC
Code:
stack=1, locals=2, args_size=2
0: aload_0
1: invokespecial #2 // Method "":()V
4: return
LineNumberTable:
line 18: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcompiler/SyntheticDemo$InnerClass;
0 5 1 x0 Lcompiler/SyntheticDemo$1;
static java.lang.String access$100(compiler.SyntheticDemo$InnerClass);
descriptor: (Lcompiler/SyntheticDemo$InnerClass;)Ljava/lang/String;
flags: ACC_STATIC, ACC_SYNTHETIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: getfield #1 // Field inner:Ljava/lang/String;
4: areturn
LineNumberTable:
line 18: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 x0 Lcompiler/SyntheticDemo$InnerClass;
}
SourceFile: "SyntheticDemo.java"
InnerClasses:
static #18; //class compiler/SyntheticDemo$1
SyntheticDemo$InnerClass里含有默认访问权限的构造器和access$100方法。并且这个构造器和方法都是synthetic的。构造器的参数是SyntheticDemo$1。access$100的逻辑是获取内部类的私有属性。
SyntheticDemo.class
在看看最后一个class，javap SyntheticDemo.class:
Compiled from "SyntheticDemo.java"
public class compiler.SyntheticDemo {
public compiler.SyntheticDemo();
public static void main(java.lang.String[]);
}
javap -v SyntheticDemo.class
Classfile /D:/idea_workspace/mycode/target/classes/compiler/SyntheticDemo.class
Last modified 2019-12-12; size 1040 bytes
MD5 checksum 7519ad7d2df356217e19642ed0a06ee4
Compiled from "SyntheticDemo.java"
public class compiler.SyntheticDemo
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #13.#32 // java/lang/Object."":()V
#2 = Class #33 // compiler/SyntheticDemo$InnerClass
#3 = Methodref #2.#34 // compiler/SyntheticDemo$InnerClass."":(Lcompiler/SyntheticDemo$1;)V
#4 = Fieldref #35.#36 // java/lang/System.out:Ljava/io/PrintStream;
#5 = Class #37 // java/lang/StringBuilder
#6 = Methodref #5.#32 // java/lang/StringBuilder."":()V
#7 = String #38 // inner:
#8 = Methodref #5.#39 // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#9 = Methodref #2.#40 // compiler/SyntheticDemo$InnerClass.access$100:(Lcompiler/SyntheticDemo$InnerClass;)Ljava/lang/String;
#10 = Methodref #5.#41 // java/lang/StringBuilder.toString:()Ljava/lang/String;
#11 = Methodref #42.#43 // java/io/PrintStream.println:(Ljava/lang/String;)V
#12 = Class #44 // compiler/SyntheticDemo
#13 = Class #45 // java/lang/Object
#14 = Class #46 // compiler/SyntheticDemo$1
#15 = Utf8 InnerClasses
#16 = Utf8 InnerClass
#17 = Utf8
#18 = Utf8 ()V
#19 = Utf8 Code
#20 = Utf8 LineNumberTable
#21 = Utf8 LocalVariableTable
#22 = Utf8 this
#23 = Utf8 Lcompiler/SyntheticDemo;
#24 = Utf8 main
#25 = Utf8 ([Ljava/lang/String;)V
#26 = Utf8 args
#27 = Utf8 [Ljava/lang/String;
#28 = Utf8 innerObject
#29 = Utf8 Lcompiler/SyntheticDemo$InnerClass;
#30 = Utf8 SourceFile
#31 = Utf8 SyntheticDemo.java
#32 = NameAndType #17:#18 // "":()V
#33 = Utf8 compiler/SyntheticDemo$InnerClass
#34 = NameAndType #17:#47 // "":(Lcompiler/SyntheticDemo$1;)V
#35 = Class #48 // java/lang/System
#36 = NameAndType #49:#50 // out:Ljava/io/PrintStream;
#37 = Utf8 java/lang/StringBuilder
#38 = Utf8 inner:
#39 = NameAndType #51:#52 // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#40 = NameAndType #53:#54 // access$100:(Lcompiler/SyntheticDemo$InnerClass;)Ljava/lang/String;
#41 = NameAndType #55:#56 // toString:()Ljava/lang/String;
#42 = Class #57 // java/io/PrintStream
#43 = NameAndType #58:#59 // println:(Ljava/lang/String;)V
#44 = Utf8 compiler/SyntheticDemo
#45 = Utf8 java/lang/Object
#46 = Utf8 compiler/SyntheticDemo$1
#47 = Utf8 (Lcompiler/SyntheticDemo$1;)V
#48 = Utf8 java/lang/System
#49 = Utf8 out
#50 = Utf8 Ljava/io/PrintStream;
#51 = Utf8 append
#52 = Utf8 (Ljava/lang/String;)Ljava/lang/StringBuilder;
#53 = Utf8 access$100
#54 = Utf8 (Lcompiler/SyntheticDemo$InnerClass;)Ljava/lang/String;
#55 = Utf8 toString
#56 = Utf8 ()Ljava/lang/String;
#57 = Utf8 java/io/PrintStream
#58 = Utf8 println
#59 = Utf8 (Ljava/lang/String;)V
{
public compiler.SyntheticDemo();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."":()V
4: return
LineNumberTable:
line 10: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcompiler/SyntheticDemo;
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=3, locals=2, args_size=1
0: new #2 // class compiler/SyntheticDemo$InnerClass
3: dup
4: aconst_null
5: invokespecial #3 // Method compiler/SyntheticDemo$InnerClass."":(Lcompiler/SyntheticDemo$1;)V
8: astore_1
9: getstatic #4 // Field java/lang/System.out:Ljava/io/PrintStream;
12: new #5 // class java/lang/StringBuilder
15: dup
16: invokespecial #6 // Method java/lang/StringBuilder."":()V
19: ldc #7 // String inner:
21: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
24: aload_1
25: invokestatic #9 // Method compiler/SyntheticDemo$InnerClass.access$100:(Lcompiler/SyntheticDemo$InnerClass;)Ljava/lang/String;
28: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
31: invokevirtual #10 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
34: invokevirtual #11 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
37: return
LineNumberTable:
line 14: 0
line 15: 9
line 16: 37
LocalVariableTable:
Start Length Slot Name Signature
0 38 0 args [Ljava/lang/String;
9 29 1 innerObject Lcompiler/SyntheticDemo$InnerClass;
}
SourceFile: "SyntheticDemo.java"
InnerClasses:
static #14; //class compiler/SyntheticDemo$1
到这里，所有谜题都解开了。在new内部类时，调用了编译器生成的那个构造器，并传递了一个null作为参数：
5: invokespecial #3 // Method compiler/SyntheticDemo$InnerClass."":(Lcompiler/SyntheticDemo$1;)V
8: astore_1
获取内部类的私有属性时，调用的编译生成的access$100方法：
invokestatic #9 // Method compiler/SyntheticDemo$InnerClass.access$100:(Lcompiler/SyntheticDemo$InnerClass;)Ljava/lang/String;解答
因为内部类是一个私有的，所以默认构造器也会是私有的构造器，那么外部类就无法调用此默认构造器。而我们知道Java语言中外部类是可以构造和访问内部类的，其实现就是编译器耍的花招，编译生成了一些构造器、类、方法，来帮我们把外部类和内部类私有部分连接起来，而这类编译器生成的构造器、方法、类就是synthetic标记的。也就是说这个特性是Java编译器赋予的，而其他运行在JVM上的语言则不一定有此特性，要看其对应的编译器是否也用了一些花招来做连接了。
如果我们给把InnerClass的inner域的private关键字去掉，再查字节码，发现内部类的access$100没有了，因为现在外部类可以直接访问到内部类的包访问权限的域，当然就不需要生成access$100来做桥梁了。读者可以自己试试。
如果我们把InnerClass的内部类private去掉，或者不去掉而是添加一个非private的内部类构造器，那么查看字节码发现，SyntheticDemo$1.class消失了，而且编译器也不生成带SyntheticDemo$1.class参数的包权限构造器了，因为不需要了。读者可以自行试验。
扩展
匿名内部类
匿名内部类用到了synthetic了吗？
public class SyntheticDemo2 {
public static void main(String[] args) {
new Thread(new Runnable() {
@Override
public void run() {
System.out.println("11");
}
}).start();
}
}以上代码也会生成一个SyntheticDemo2$1.class。我们查看其字节码：
Classfile /D:/idea_workspace/mycode/target/classes/compiler/SyntheticDemo2$1.class
Last modified 2019-12-12; size 503 bytes
MD5 checksum 207c9ba26f66c2fbfd5d092c7f9f844e
Compiled from "SyntheticDemo2.java"
final class compiler.SyntheticDemo2$1 implements java.lang.Runnable
minor version: 0
major version: 52
flags: ACC_FINAL, ACC_SUPER
Constant pool:
#1 = Methodref #3.#19 // java/lang/Object."":()V
#2 = Class #20 // compiler/SyntheticDemo2$1
#3 = Class #21 // java/lang/Object
#4 = Class #22 // java/lang/Runnable
#5 = Utf8
#6 = Utf8 ()V
#7 = Utf8 Code
#8 = Utf8 LineNumberTable
#9 = Utf8 LocalVariableTable
#10 = Utf8 this
#11 = Utf8 InnerClasses
#12 = Utf8 Lcompiler/SyntheticDemo2$1;
#13 = Utf8 run
#14 = Utf8 SourceFile
#15 = Utf8 SyntheticDemo2.java
#16 = Utf8 EnclosingMethod
#17 = Class #23 // compiler/SyntheticDemo2
#18 = NameAndType #24:#25 // main:([Ljava/lang/String;)V
#19 = NameAndType #5:#6 // "":()V
#20 = Utf8 compiler/SyntheticDemo2$1
#21 = Utf8 java/lang/Object
#22 = Utf8 java/lang/Runnable
#23 = Utf8 compiler/SyntheticDemo2
#24 = Utf8 main
#25 = Utf8 ([Ljava/lang/String;)V
{
compiler.SyntheticDemo2$1();
descriptor: ()V
flags:
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."":()V
4: return
LineNumberTable:
line 13: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcompiler/SyntheticDemo2$1;
public void run();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=0, locals=1, args_size=1
0: return
LineNumberTable:
line 17: 0
LocalVariableTable:
Start Length Slot Name Signature
0 1 0 this Lcompiler/SyntheticDemo2$1;
}
SourceFile: "SyntheticDemo2.java"
EnclosingMethod: #17.#18 // compiler.SyntheticDemo2.main
InnerClasses:
static #2; //class compiler/SyntheticDemo2$1
字节码中没有ACC_SYNTHETIC标记，说明其与 synthetic 无关。
lambda 表达式
class SyntheticDemo3 {
public static void main(String[] args) {
new Thread(()->{}).start();
}
}只有一个SyntheticDemo3.class
其字节码：
Classfile /D:/idea_workspace/mycode/target/classes/compiler/SyntheticDemo3.class
Last modified 2019-12-12; size 1042 bytes
MD5 checksum 50722aaf911c908da83df897f5141c05
Compiled from "SyntheticDemo3.java"
class compiler.SyntheticDemo3
minor version: 0
major version: 52
flags: ACC_SUPER
Constant pool:
#1 = Methodref #7.#22 // java/lang/Object."":()V
#2 = Class #23 // java/lang/Thread
#3 = InvokeDynamic #0:#28 // #0:run:()Ljava/lang/Runnable;
#4 = Methodref #2.#29 // java/lang/Thread."":(Ljava/lang/Runnable;)V
#5 = Methodref #2.#30 // java/lang/Thread.start:()V
#6 = Class #31 // compiler/SyntheticDemo3
#7 = Class #32 // java/lang/Object
#8 = Utf8
#9 = Utf8 ()V
#10 = Utf8 Code
#11 = Utf8 LineNumberTable
#12 = Utf8 LocalVariableTable
#13 = Utf8 this
#14 = Utf8 Lcompiler/SyntheticDemo3;
#15 = Utf8 main
#16 = Utf8 ([Ljava/lang/String;)V
#17 = Utf8 args
#18 = Utf8 [Ljava/lang/String;
#19 = Utf8 lambda$main$0
#20 = Utf8 SourceFile
#21 = Utf8 SyntheticDemo3.java
#22 = NameAndType #8:#9 // "":()V
#23 = Utf8 java/lang/Thread
#24 = Utf8 BootstrapMethods
#25 = MethodHandle #6:#33 // invokestatic java/lang/invoke/LambdaMetafactory.metafactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke
/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
#26 = MethodType #9 // ()V
#27 = MethodHandle #6:#34 // invokestatic compiler/SyntheticDemo3.lambda$main$0:()V
#28 = NameAndType #35:#36 // run:()Ljava/lang/Runnable;
#29 = NameAndType #8:#37 // "":(Ljava/lang/Runnable;)V
#30 = NameAndType #38:#9 // start:()V
#31 = Utf8 compiler/SyntheticDemo3
#32 = Utf8 java/lang/Object
#33 = Methodref #39.#40 // java/lang/invoke/LambdaMetafactory.metafactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;L
java/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
#34 = Methodref #6.#41 // compiler/SyntheticDemo3.lambda$main$0:()V
#35 = Utf8 run
#36 = Utf8 ()Ljava/lang/Runnable;
#37 = Utf8 (Ljava/lang/Runnable;)V
#38 = Utf8 start
#39 = Class #42 // java/lang/invoke/LambdaMetafactory
#40 = NameAndType #43:#47 // metafactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/l
ang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
#41 = NameAndType #19:#9 // lambda$main$0:()V
#42 = Utf8 java/lang/invoke/LambdaMetafactory
#43 = Utf8 metafactory
#44 = Class #49 // java/lang/invoke/MethodHandles$Lookup
#45 = Utf8 Lookup
#46 = Utf8 InnerClasses
#47 = Utf8 (Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/
lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
#48 = Class #50 // java/lang/invoke/MethodHandles
#49 = Utf8 java/lang/invoke/MethodHandles$Lookup
#50 = Utf8 java/lang/invoke/MethodHandles
{
compiler.SyntheticDemo3();
descriptor: ()V
flags:
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."":()V
4: return
LineNumberTable:
line 8: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcompiler/SyntheticDemo3;
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=3, locals=1, args_size=1
0: new #2 // class java/lang/Thread
3: dup
4: invokedynamic #3, 0 // InvokeDynamic #0:run:()Ljava/lang/Runnable;
9: invokespecial #4 // Method java/lang/Thread."":(Ljava/lang/Runnable;)V
12: invokevirtual #5 // Method java/lang/Thread.start:()V
15: return
LineNumberTable:
line 11: 0
line 12: 15
LocalVariableTable:
Start Length Slot Name Signature
0 16 0 args [Ljava/lang/String;
}
SourceFile: "SyntheticDemo3.java"
InnerClasses:
public static final #45= #44 of #48; //Lookup=class java/lang/invoke/MethodHandles$Lookup of class java/lang/invoke/MethodHandles
BootstrapMethods:
0: #25 invokestatic java/lang/invoke/LambdaMetafactory.metafactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/Method
Type;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
Method arguments:
#26 ()V
#27 invokestatic compiler/SyntheticDemo3.lambda$main$0:()V
#26 ()V
lambda表达式没有通过内部类实现，而是通过invokedynamic这个指令解决的。有机会写一遍invokedynamic的文章。
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