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android 内部类的优化

developer.android.com 文档中有一篇关于性能的文章,里面提到了内部类的使用。文章建议“对于私有内部类 使用 包訪问权限取代私有权限訪问”,

这里说的是在内部类訪问外部类的成员或方法时假设 内部类是私有的而且外部类的成员也是私有的,那么编译器就会为内部类在外部类中添加一个静态方法。

真的是这种吗?仅仅有试一试才知道。

我们使用一个简单的样例来測试下:

public class One {

	private int a;
	
	private class B{
		
		public int getA(){
			return a;
		}
	}

}

上边的代码为One创建了一个私有的内部类B,而且B中有一个方法訪问到了 One中的私有成员 a。我们把上边的代码编译成 dex文件后导出程序的指令码与类信息。

导出的结果例如以下:

Processing ‘one.dex‘...
Opened ‘one.dex‘, DEX version ‘035‘
Class #0            -
  Class descriptor  : ‘LOne$B;‘
  Access flags      : 0x0000 ()
  Superclass        : ‘Ljava/lang/Object;‘
  Interfaces        -
  Static fields     -
  Instance fields   -
    #0              : (in LOne$B;)
      name          : ‘this$0‘
      type          : ‘LOne;‘
      access        : 0x1010 (FINAL SYNTHETIC)
  Direct methods    -
    #0              : (in LOne$B;)
      name          : ‘<init>‘
      type          : ‘(LOne;)V‘
      access        : 0x10002 (PRIVATE CONSTRUCTOR)
      code          -
      registers     : 2
      ins           : 2
      outs          : 1
      insns size    : 6 16-bit code units
0001a0:                                        |[0001a0] One.B.<init>:(LOne;)V
0001b0: 5b01 0000                              |0000: iput-object v1, v0, LOne$B;.this$0:LOne; // field@0000
0001b4: 7010 0400 0000                         |0002: invoke-direct {v0}, Ljava/lang/Object;.<init>:()V // method@0004
0001ba: 0e00                                   |0005: return-void
      catches       : (none)
      positions     : 
        0x0000 line=7
      locals        : 
        0x0000 - 0x0006 reg=0 this LOne$B; 

  Virtual methods   -
    #0              : (in LOne$B;)
      name          : ‘getA‘
      type          : ‘()I‘
      access        : 0x0001 (PUBLIC)
      code          -
      registers     : 2
      ins           : 1
      outs          : 1
      insns size    : 7 16-bit code units
0001bc:                                        |[0001bc] One.B.getA:()I
0001cc: 5410 0000                              |0000: iget-object v0, v1, LOne$B;.this$0:LOne; // field@0000
0001d0: 7110 0300 0000                         |0002: invoke-static {v0}, LOne;.access$0:(LOne;)I // method@0003
0001d6: 0a00                                   |0005: move-result v0
0001d8: 0f00                                   |0006: return v0
      catches       : (none)
      positions     : 
        0x0000 line=10
      locals        : 
        0x0000 - 0x0007 reg=1 this LOne$B; 

  source_file_idx   : 10 (One.java)

Class #1            -
  Class descriptor  : ‘LOne;‘
  Access flags      : 0x0001 (PUBLIC)
  Superclass        : ‘Ljava/lang/Object;‘
  Interfaces        -
  Static fields     -
  Instance fields   -
    #0              : (in LOne;)
      name          : ‘a‘
      type          : ‘I‘
      access        : 0x0002 (PRIVATE)
  Direct methods    -
    #0              : (in LOne;)
      name          : ‘<init>‘
      type          : ‘()V‘
      access        : 0x10001 (PUBLIC CONSTRUCTOR)
      code          -
      registers     : 1
      ins           : 1
      outs          : 1
      insns size    : 4 16-bit code units
0001dc:                                        |[0001dc] One.<init>:()V
0001ec: 7010 0400 0000                         |0000: invoke-direct {v0}, Ljava/lang/Object;.<init>:()V // method@0004
0001f2: 0e00                                   |0003: return-void
      catches       : (none)
      positions     : 
        0x0000 line=3
      locals        : 
        0x0000 - 0x0004 reg=0 this LOne; 

    #1              : (in LOne;)
      name          : ‘access$0‘
      type          : ‘(LOne;)I‘
      access        : 0x1008 (STATIC SYNTHETIC)
      code          -
      registers     : 2
      ins           : 1
      outs          : 0
      insns size    : 3 16-bit code units
0001f4:                                        |[0001f4] One.access$0:(LOne;)I
000204: 5210 0100                              |0000: iget v0, v1, LOne;.a:I // field@0001
000208: 0f00                                   |0002: return v0
      catches       : (none)
      positions     : 
        0x0000 line=5
      locals        : 

  Virtual methods   -
  source_file_idx   : 10 (One.java)

哈看到没 One中多一个方法,而我们的源代码中是没有的。
One.access$0:(LOne;)I

如今我们把源代码改下

public class One {

	private int a;
	
	class B{
		
		public int getA(){
			return a;
		}
	}

}

然后在编译,导出:

Processing ‘one.dex‘...
Opened ‘one.dex‘, DEX version ‘035‘
Class #0            -
  Class descriptor  : ‘LOne$B;‘
  Access flags      : 0x0000 ()
  Superclass        : ‘Ljava/lang/Object;‘
  Interfaces        -
  Static fields     -
  Instance fields   -
    #0              : (in LOne$B;)
      name          : ‘this$0‘
      type          : ‘LOne;‘
      access        : 0x1010 (FINAL SYNTHETIC)
  Direct methods    -
    #0              : (in LOne$B;)
      name          : ‘<init>‘
      type          : ‘(LOne;)V‘
      access        : 0x10000 (CONSTRUCTOR)
      code          -
      registers     : 2
      ins           : 2
      outs          : 1
      insns size    : 6 16-bit code units
0001a0:                                        |[0001a0] One.B.<init>:(LOne;)V
0001b0: 5b01 0000                              |0000: iput-object v1, v0, LOne$B;.this$0:LOne; // field@0000
0001b4: 7010 0400 0000                         |0002: invoke-direct {v0}, Ljava/lang/Object;.<init>:()V // method@0004
0001ba: 0e00                                   |0005: return-void
      catches       : (none)
      positions     : 
        0x0000 line=7
      locals        : 
        0x0000 - 0x0006 reg=0 this LOne$B; 

  Virtual methods   -
    #0              : (in LOne$B;)
      name          : ‘getA‘
      type          : ‘()I‘
      access        : 0x0001 (PUBLIC)
      code          -
      registers     : 2
      ins           : 1
      outs          : 1
      insns size    : 7 16-bit code units
0001bc:                                        |[0001bc] One.B.getA:()I
0001cc: 5410 0000                              |0000: iget-object v0, v1, LOne$B;.this$0:LOne; // field@0000
0001d0: 7110 0300 0000                         |0002: invoke-static {v0}, LOne;.access$0:(LOne;)I // method@0003
0001d6: 0a00                                   |0005: move-result v0
0001d8: 0f00                                   |0006: return v0
      catches       : (none)
      positions     : 
        0x0000 line=10
      locals        : 
        0x0000 - 0x0007 reg=1 this LOne$B; 

  source_file_idx   : 10 (One.java)

Class #1            -
  Class descriptor  : ‘LOne;‘
  Access flags      : 0x0001 (PUBLIC)
  Superclass        : ‘Ljava/lang/Object;‘
  Interfaces        -
  Static fields     -
  Instance fields   -
    #0              : (in LOne;)
      name          : ‘a‘
      type          : ‘I‘
      access        : 0x0002 (PRIVATE)
  Direct methods    -
    #0              : (in LOne;)
      name          : ‘<init>‘
      type          : ‘()V‘
      access        : 0x10001 (PUBLIC CONSTRUCTOR)
      code          -
      registers     : 1
      ins           : 1
      outs          : 1
      insns size    : 4 16-bit code units
0001dc:                                        |[0001dc] One.<init>:()V
0001ec: 7010 0400 0000                         |0000: invoke-direct {v0}, Ljava/lang/Object;.<init>:()V // method@0004
0001f2: 0e00                                   |0003: return-void
      catches       : (none)
      positions     : 
        0x0000 line=3
      locals        : 
        0x0000 - 0x0004 reg=0 this LOne; 

    #1              : (in LOne;)
      name          : ‘access$0‘
      type          : ‘(LOne;)I‘
      access        : 0x1008 (STATIC SYNTHETIC)
      code          -
      registers     : 2
      ins           : 1
      outs          : 0
      insns size    : 3 16-bit code units
0001f4:                                        |[0001f4] One.access$0:(LOne;)I
000204: 5210 0100                              |0000: iget v0, v1, LOne;.a:I // field@0001
000208: 0f00                                   |0002: return v0
      catches       : (none)
      positions     : 
        0x0000 line=5
      locals        : 

  Virtual methods   -
  source_file_idx   : 10 (One.java)
依旧有附加的方法
One.access$0:(LOne;)I


我们在改动下

public class One {

	int a;
	
	class B{
		
		public int getA(){
			return a;
		}
	}

}

在编译 导出

Processing ‘one.dex‘...
Opened ‘one.dex‘, DEX version ‘035‘
Class #0            -
  Class descriptor  : ‘LOne$B;‘
  Access flags      : 0x0000 ()
  Superclass        : ‘Ljava/lang/Object;‘
  Interfaces        -
  Static fields     -
  Instance fields   -
    #0              : (in LOne$B;)
      name          : ‘this$0‘
      type          : ‘LOne;‘
      access        : 0x1010 (FINAL SYNTHETIC)
  Direct methods    -
    #0              : (in LOne$B;)
      name          : ‘<init>‘
      type          : ‘(LOne;)V‘
      access        : 0x10000 (CONSTRUCTOR)
      code          -
      registers     : 2
      ins           : 2
      outs          : 1
      insns size    : 6 16-bit code units
000184:                                        |[000184] One.B.<init>:(LOne;)V
000194: 5b01 0000                              |0000: iput-object v1, v0, LOne$B;.this$0:LOne; // field@0000
000198: 7010 0300 0000                         |0002: invoke-direct {v0}, Ljava/lang/Object;.<init>:()V // method@0003
00019e: 0e00                                   |0005: return-void
      catches       : (none)
      positions     : 
        0x0000 line=7
      locals        : 
        0x0000 - 0x0006 reg=0 this LOne$B; 

  Virtual methods   -
    #0              : (in LOne$B;)
      name          : ‘getA‘
      type          : ‘()I‘
      access        : 0x0001 (PUBLIC)
      code          -
      registers     : 2
      ins           : 1
      outs          : 0
      insns size    : 5 16-bit code units
0001a0:                                        |[0001a0] One.B.getA:()I
0001b0: 5410 0000                              |0000: iget-object v0, v1, LOne$B;.this$0:LOne; // field@0000
0001b4: 5200 0100                              |0002: iget v0, v0, LOne;.a:I // field@0001
0001b8: 0f00                                   |0004: return v0
      catches       : (none)
      positions     : 
        0x0000 line=10
      locals        : 
        0x0000 - 0x0005 reg=1 this LOne$B; 

  source_file_idx   : 9 (One.java)

Class #1            -
  Class descriptor  : ‘LOne;‘
  Access flags      : 0x0001 (PUBLIC)
  Superclass        : ‘Ljava/lang/Object;‘
  Interfaces        -
  Static fields     -
  Instance fields   -
    #0              : (in LOne;)
      name          : ‘a‘
      type          : ‘I‘
      access        : 0x0000 ()
  Direct methods    -
    #0              : (in LOne;)
      name          : ‘<init>‘
      type          : ‘()V‘
      access        : 0x10001 (PUBLIC CONSTRUCTOR)
      code          -
      registers     : 1
      ins           : 1
      outs          : 1
      insns size    : 4 16-bit code units
0001bc:                                        |[0001bc] One.<init>:()V
0001cc: 7010 0300 0000                         |0000: invoke-direct {v0}, Ljava/lang/Object;.<init>:()V // method@0003
0001d2: 0e00                                   |0003: return-void
      catches       : (none)
      positions     : 
        0x0000 line=3
      locals        : 
        0x0000 - 0x0004 reg=0 this LOne; 

  Virtual methods   -
  source_file_idx   : 9 (One.java)

已经没有附加的方法了。

假设外部类的成员是包訪问的,内部类是私有的相同也不会产生附加的方法。

内部类对外部类成员的訪问是不能直接訪问私有成员的,编译器会添加额外的辅助方法,避免的方法是改动外部类的成员为包訪问,文档中也提到了这会在一定程度上破坏

封装。


android 内部类的优化