今天在浏览知乎的时候，看到了这个问题，感觉很多人说的不清楚。问题链接：Java 类名.class与类名.this 的区别?
话说它有什么区别呢？从API层面上来说，"类.class"返回该类所对应的class对象，而"类.this"得到的是该类的对象，这两者的区别大着呢！前者是描述该类的Class对象，后者是该类的实例对象，两者没有可比性。API层面上有些说不清，还是从字节码层面上来说吧！话不多说，先上车。
类.class
看“类.class”
public class Demo62 {
public static void main(String[] args) {
Class clz = Demo62.class;
}
}
执行反编译：
"D:Program FilesJavajdk1.8.0_77injavap.exe" -v -p com.bigdata.java.Demo62
Classfile /D:/Project/JvmDemo/target/classes/com/bigdata/java/Demo62.class
Last modified 2020-2-17; size 438 bytes
MD5 checksum 19a4a4e0310212eaabf8f8a5c18d82fc
Compiled from "Demo62.java"
public class com.bigdata.java.Demo62
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #3.#19 // java/lang/Object."<init>":()V
#2 = Class #20 // com/bigdata/java/Demo62
#3 = Class #21 // java/lang/Object
#4 = Utf8 <init>
#5 = Utf8 ()V
#6 = Utf8 Code
#7 = Utf8 LineNumberTable
#8 = Utf8 LocalVariableTable
#9 = Utf8 this
#10 = Utf8 Lcom/bigdata/java/Demo62;
#11 = Utf8 main
#12 = Utf8 ([Ljava/lang/String;)V
#13 = Utf8 args
#14 = Utf8 [Ljava/lang/String;
#15 = Utf8 clz
#16 = Utf8 Ljava/lang/Class;
#17 = Utf8 SourceFile
#18 = Utf8 Demo62.java
#19 = NameAndType #4:#5 // "<init>":()V
#20 = Utf8 com/bigdata/java/Demo62
#21 = Utf8 java/lang/Object
{
public com.bigdata.java.Demo62();
descriptor: ()V
flags: ACC_PUBLIC
Code:
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
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcom/bigdata/java/Demo62;
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=1, locals=2, args_size=1
0: ldc #2 // class com/bigdata/java/Demo62
2: astore_1
3: return
LineNumberTable:
line 5: 0
line 6: 3
LocalVariableTable:
Start Length Slot Name Signature
0 4 0 args [Ljava/lang/String;
3 1 1 clz Ljava/lang/Class;
}
SourceFile: "Demo62.java"
Process finished with exit code 0
这一小段代码编译后这么多，别被吓到，有用的没几条，着重看这些：
Code:
stack=1, locals=2, args_size=1
0: ldc #2 // class com/bigdata/java/Demo62
2: astore_1
3: return
“ 0: ldc #2 // class com/bigdata/java/Demo62”，表示将常量池中索引2位置上的元素，压入到操作数的栈顶。通过查阅常量池，我们可以看到它就是“com/bigdata/java/Demo62”，只是它是一个符号引用，在类的解析阶段被转换为了直接引用，因此这里压入到操作数栈顶的是解析后的直接引用（也即该类所对应的Class对象的地址或指针），
https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-6.html#jvms-6.5.ldc
“astore_1”，弹出操作数栈顶的值，然后放到局部变量表的slot1中。
所以最终看到的就是局部变量表的slot1中有一个class对象
LocalVariableTable:
Start Length Slot Name Signature
0 4 0 args [Ljava/lang/String;
3 1 1 clz Ljava/lang/Class;
小结：所以“类.class”返回的实际上就是该类所对应的class对象。但是底层实际上是通过常量池找到对应的class对象的。
类.this
再看“类.this”
public class Demo62 {
public Demo62 method1(){
return Demo62.this;
}
}
执行反编译：
"D:Program FilesJavajdk1.8.0_77injavap.exe" -v -p com.bigdata.java.Demo62
Classfile /D:/Project/JvmDemo/target/classes/com/bigdata/java/Demo62.class
Last modified 2020-2-17; size 375 bytes
MD5 checksum 0ca3c6df6b1bc2a64c91aa755eba16fe
Compiled from "Demo62.java"
public class com.bigdata.java.Demo62
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #3.#15 // java/lang/Object."<init>":()V
#2 = Class #16 // com/bigdata/java/Demo62
#3 = Class #17 // java/lang/Object
#4 = Utf8 <init>
#5 = Utf8 ()V
#6 = Utf8 Code
#7 = Utf8 LineNumberTable
#8 = Utf8 LocalVariableTable
#9 = Utf8 this
#10 = Utf8 Lcom/bigdata/java/Demo62;
#11 = Utf8 method1
#12 = Utf8 ()Lcom/bigdata/java/Demo62;
#13 = Utf8 SourceFile
#14 = Utf8 Demo62.java
#15 = NameAndType #4:#5 // "<init>":()V
#16 = Utf8 com/bigdata/java/Demo62
#17 = Utf8 java/lang/Object
{
public com.bigdata.java.Demo62();
descriptor: ()V
flags: ACC_PUBLIC
Code:
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
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcom/bigdata/java/Demo62;
public com.bigdata.java.Demo62 method1();
descriptor: ()Lcom/bigdata/java/Demo62;
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: areturn
LineNumberTable:
line 5: 0
LocalVariableTable:
Start Length Slot Name Signature
0 2 0 this Lcom/bigdata/java/Demo62;
}
SourceFile: "Demo62.java"
Process finished with exit code 0
重点关注这段代码
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: areturn
LocalVariableTable:
Start Length Slot Name Signature
0 2 0 this Lcom/bigdata/java/Demo62;
"aload_0"，表示将加载局部变量表slot0中的元素到操作数栈顶，而这个slot0中存储的就是this。这里先说明一下，每个方法（非静态方法）都隐含有一个“this”参数，这也是为什么我们能够在方法中使用this的原因。
“areturn”，表示从方法返回引用。这里它将弹出操作数栈弹出的元素并返回。
小结：“类.this”得到的是当前类的实例。底层实际上就是得到方法的隐含参数this。
最后，
public class Demo62 {
public static void main(String[] args) {
Class clz = Demo62.class;
Demo62 instance1 = new Demo62();
Demo62 instance2 = instance1.method1();
System.out.println(instance1 == instance2);//true
System.out.println(instance1.getClass()==clz);//true
}
public Demo62 method1(){
return Demo62.this;
}
}
通过上面的分析，得到这样的执行结果是毫不意外的。
再看另外热议的问题
class Foo {
class Bar {
Foo getFoo() {
return Foo.this;
}
}
}
在Foo.Bar类中的getFoo()方法中，如果直接写“this”的话所指的是这个Foo.Bar类的实例，而如果要指定外围的Foo类的this实例的话，这里就得写成Foo.this。
特别的，如果在上例的getFoo()方法中写Bar.this的话，作用就跟直接写this一样，指定的是当前的Foo.Bar类实例。作者：RednaxelaFX
链接：https://www.zhihu.com/question/55565290/answer/145355951
来源：知乎
著作权归作者所有。商业转载请联系作者获得授权，非商业转载请注明出处。
下面我们尝试从字节码的角度来解读，话不多说，上码：
...
Constant pool:
#1 = Fieldref #3.#20 // com/bigdata/java/other/Foo$Bar.this$0:Lcom/bigdata/java/other/Foo;
#2 = Methodref #4.#21 // java/lang/Object."<init>":()V
#3 = Class #23 // com/bigdata/java/other/Foo$Bar
#4 = Class #24 // java/lang/Object
#5 = Utf8 this$0
#6 = Utf8 Lcom/bigdata/java/other/Foo;
#7 = Utf8 <init>
#8 = Utf8 (Lcom/bigdata/java/other/Foo;)V
#9 = Utf8 Code
#10 = Utf8 LineNumberTable
#11 = Utf8 LocalVariableTable
#12 = Utf8 this
#13 = Utf8 Bar
#14 = Utf8 InnerClasses
#15 = Utf8 Lcom/bigdata/java/other/Foo$Bar;
#16 = Utf8 getFoo
#17 = Utf8 ()Lcom/bigdata/java/other/Foo;
#18 = Utf8 SourceFile
#19 = Utf8 Foo.java
#20 = NameAndType #5:#6 // this$0:Lcom/bigdata/java/other/Foo;
#21 = NameAndType #7:#25 // "<init>":()V
#22 = Class #26 // com/bigdata/java/other/Foo
#23 = Utf8 com/bigdata/java/other/Foo$Bar
#24 = Utf8 java/lang/Object
#25 = Utf8 ()V
#26 = Utf8 com/bigdata/java/other/Foo
...
{
final com.bigdata.java.other.Foo this$0;
descriptor: Lcom/bigdata/java/other/Foo;
flags: ACC_FINAL, ACC_SYNTHETIC
com.bigdata.java.other.Foo$Bar(com.bigdata.java.other.Foo);
descriptor: (Lcom/bigdata/java/other/Foo;)V
flags:
Code:
stack=2, locals=2, args_size=2
0: aload_0
1: aload_1
2: putfield #1 // Field this$0:Lcom/bigdata/java/other/Foo;
5: aload_0
6: invokespecial #2 // Method java/lang/Object."<init>":()V
9: return
LineNumberTable:
line 4: 0
LocalVariableTable:
Start Length Slot Name Signature
0 10 0 this Lcom/bigdata/java/other/Foo$Bar;
0 10 1 this$0 Lcom/bigdata/java/other/Foo;
com.bigdata.java.other.Foo getFoo();
descriptor: ()Lcom/bigdata/java/other/Foo;
flags:
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: getfield #1 // Field this$0:Lcom/bigdata/java/other/Foo;
4: areturn
LineNumberTable:
line 6: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcom/bigdata/java/other/Foo$Bar;
}
...
重点看“ getFoo()”方法
aload_0：表示将本地变量表中的slot0元素加载到操作数栈顶，这里也就是将this（com/bigdata/java/other/Foo$Bar）压入到操作数栈顶。
getfield：表示获取字段。弹出操作数栈顶的com/bigdata/java/other/Foo$Bar，然后获取该对象的#1索引所对应的常量池的值，也即“this$0”，它的类型是“Foo”，然后将它压入到操作数的栈顶
https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-6.html#jvms-6.5.getfield
areturn：弹出操作数栈顶的值并返回，也即返回“this.$0”，它的类型是Foo。
小结：通过这个实例，印证了Foo.this返回的就是Foo。
下面我们在分别看Bar.this和this
脑洞大开，写成这种形式：
class Foo {
class Bar {
Foo getFoo() {
return this;//编译器报错，提示类型不兼容
}
}
}
这里的类型不兼容也不难理解，由于getFoo中的this，指的是“Foo$Bar”，而返回值是“Foo”。
修改为return this
下面将返回值修改为this
class Foo {
class Bar {
Bar getFoo() {
return this;
}
}
}
反编译：
...
com.bigdata.java.other.Foo$Bar getFoo();
descriptor: ()Lcom/bigdata/java/other/Foo$Bar;
flags:
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: areturn
LineNumberTable:
line 6: 0
LocalVariableTable:
Start Length Slot Name Signature
0 2 0 this Lcom/bigdata/java/other/Foo$Bar;
...
可以发现，它返回了本地变量表slot0的元素，也即"Foo$Bar"
修改为Bar.this
class Foo {
class Bar {
Bar getFoo() {
return Bar.this;
}
}
}
反编译：
...
com.bigdata.java.other.Foo$Bar getFoo();
descriptor: ()Lcom/bigdata/java/other/Foo$Bar;
flags:
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: areturn
LineNumberTable:
line 6: 0
LocalVariableTable:
Start Length Slot Name Signature
0 2 0 this Lcom/bigdata/java/other/Foo$Bar;
...
可以发现字节码和return this是一样的。返回值也是一样的。从而印证了“getFoo()方法中写Bar.this的话，作用就跟直接写this一样，指定的是当前的Foo.Bar类实例”的结论。