[javap]为什么对象被new之后要再执行dup操作?

最新推荐文章于 2022-11-22 10:59:12 发布

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最新推荐文章于 2022-11-22 10:59:12 发布

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本文详细介绍了Java字节码指令dup的功能和用法，该指令用于复制操作数栈顶的单字项，并将其再次压入栈中。通过一个具体的例子展示了如何使用dup指令来创建对象引用的副本。

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dup

duplicate top single-word item on the stack

1.Stack

Before	After
item	item
…	item
	…

2.Description

This pops the top single-word value off the operand stack, and then pushes that value twice - i.e. it makes an extra copy of the top item on the stack.

This instruction cannot be used to duplicate two-word items (longs or doubles) - use dup2 instead.

3.Example

; This is like the java expression:
;
;   StringBuffer x = new StringBuffer();
;

; Make a new StringBuffer object and leave a reference to it
; on the stack:
new java/lang/StringBuffer

; [ Stack now contains: objectref ]

; Duplicate the object reference:
dup

; [ Stack now contains: objectref objectref ]

; Invoke the object's initializer:
invokespecial java/lang/StringBuffer/<init>()V

; [ Stack now contains: objectref ]

; Store the objectref in local variable 1.
astore_1

; [ Stack is now empty. ]