161.

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putstatic

set value of static field

Jasmin Syntax

     putstatic <field-spec> <descriptor> 

<field-spec> is composed of two parts, a classname and a fieldname. The classname is all of the characters in the <field-spec> up to the last '/' character, and the fieldname is the rest of the characters after the last '/'. For example:

     foo/baz/AnotherClass/aVeryLongFieldName     -- classname -------/-- fieldname ----- 

{{JM - can we turn this into a nicer diagram?}}

 <descriptor> is the Java type descriptor for the field, for example Ljava/io/PrintStream; 

In Jasmin, the .field directive is used to add a field to a class. See Chapter 16 for a description of this and other Jasmin directives.

Stack

Before	After
value	...
...

or, for static fields holding longs or doubles:

Before	After
value-word1	...
value-word2
...

Description

putstatic sets the value of the static field (also known as a class field) identified by <field-spec> to the single or double word value on the operand stack. For example, when you write the Java expression:

     java.lang.System.out = myStream; 

this generates a call to getstatic like:

     aload_1    ; push object reference in local variable 1 (i.e. myStream) onto stack                ; now use putstatic to assign it to System.out     putstatic java/lang/System/out Ljava/io/PrintStream; 

In Jasmin, putstatic takes two parameters, <field-spec> and <descriptor>. <field-spec> provides classname, the name of the class that defines the static field, as well fieldname, as the name of the field. In the example above, the <field-spec> is "java/lang/System/out", indicating that the classname is "java/lang/System" and the fieldname is "out". <descriptor> indicates the type of data held in the field, and is a standard Java type descriptor (see Chapter 4). In the example above, <descriptor> is "Ljava/io/PrintStream;", i.e. an instance of the PrintStream class.

putstatic first resolves classname into a Java class. Then it locates the fieldname in that class, determining the width of the field (in bytes) and its offset (in bytes) from the base of the class's static data. The type of the field must match <descriptor>. See Chapter 7 for more on how fields are resolved

To set the value of the field, putstatic pops either a 4-byte or 8-byte quantity off the stack (depending on the field descriptor), and truncates it to be wodth bytes long. Then it sets the bytes starting at offset and extending for width bytes in the class's static data to the new value.

Bytecode

In bytecode, the putstatic opcode is followed by a 16-bit unsigned integer index. This is the index of an entry in the constant pool of the current class. The entry is tagged a CONSTANT_Fieldref entry. The fieldref entry lists a CONSTANT_Class entry in the constant pool whose name is the classname given in <field-spec>, as well as a CONSTANT_NameAndType entry in the constant pool, whose name is the fieldname given in <field-spec>, and whose descriptor is the string given by <descriptor>.

Type	Description
u1	putstatic opcode = 0xB3 (179)
u2	index

See Also

putfield, getfield, getstatic

Notes

Fields cannot be overriden, although they can be 'shadowed'. For example, with the two classes:

     class A { static int X; } and     class B extends A { static int X; } 

then the runtime system will allocate storage for both the static field "A/X" and the static field "B/X". Which field is accessed is determined by the class name given in <field-spec>.

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Java Virtual Machine, by Jon Meyer and Troy Downing, O'Reilly Associates