关于“类.class”和“类.this”

Posted 2021-03-09 cosmos-wong

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。

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。

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。