What is RIWO (Read Indirectly Write Out) state

问题

I was reading about the static flow control and came across the RIWO concept. Can someone explain this with simple terminology and perhaps a code sample?

This is related to the error "Illegal forward reference".

Relevant link.

回答1:

After going through some material and discussing with couple of guys offline i found out the following information.

When a java class is getting executed there are few steps which JVM performs few steps sequentially.

Identify the static members from top to bottom.
Executes static variables assignments and static blocks from top to bottom.
Executes the main method.

During these phases there is one such state called RIWO(Read Indirectly Write Only) for a static variable.

During RIWO a variable cannot be accessed directly with its reference. Instead we need to use an indirect way to call certain variables.

for example:

class Riwo
{
 static int i = 10;
 static
 {
  System.out.println(i);
 }
 }

In the above case the output is 10.

class Riwo {
static int i = 10;
static {
    m1();
    System.out.println("block1");
}

public static void main(String... args) {
    m1();
    System.out.println("block main");
}

public static void m1() {
    System.out.println(j);
    System.out.println("block m1");
}

static int j = 20;
static {
    System.out.println("end of code");
}
}

In the above case the output is

0 block m1 block1 end of code 20 block m1 block main

class Riwo
{
static
{
    System.out.println(i);
    System.out.println("block1");
}
static int i = 10;
public static void main(String... args)
{
    System.out.println("main block");
}
}

In the above case we get the following compile time error

Riwo.java:5: illegal forward reference System.out.println(i);

That means we cannot read a static variable directly when it is in RIWO state.We should call the variable indirectly using a method.

回答2:

Static variable have two states

RIWO (Read Indirectly Write Only)
RW(Read Write)
.

RIWO: Before assigning original value to static variable, variable state is called as RIWO state.

RW: After assigning original value to static variable, variable state is called as RW state.

Static variable have two type of reading

Direct Read
InDirect Read

Direct Read:
Reading static variable from static block directly is called direct reading.

InDirect Reading:
Reading static variable from static block by calling another method is called indirect reading.

Direct Read Example:
Note: when static variable in RIWO state we cant perform Direct Reading.

class DirectReadingExample{
          static {
                    System.out.println("i==="+i);
                 }
                 //i is in RIWO state
                 static int i=100;
                 //i is in RW State
    }

Output: compile time Error:illegal forward reference.

InDirect Read Example:
Note: when static variable in RIWO state we can perform InDirect Reading.

class InDirectReadingExample{
          static {
                    m1();
                 }
          static m1(){
                  System.out.println("i===="+i);
          }
                 //i is in RIWO state
                 static int i=100;
                 //i is in RW State
    }

Output: i===0

回答3:

if a variable is just identified by the JVM and original value is not yet assigned then the variable is said to be in RIWO

_______________________that is run without error __________________________________

   class Test{
        static int num=10;
        static{
            System.out.print(num);
            System.exit(0);
        }
    }

__________________________that is run within error______________________________

class Test{

    static{
        System.out.print(num);

    }
    static int num=10;

}

___________________that is run without error____________________

class Test{

    static{
        int x=readVar();
    }
    static int readVar(){
        System.out.print(num);
        return 0;
    }
    static int  num=10;

}

num is identified by JVM 1st step then default value is given at that time so that value of num variable is 0 because we can't get value directly but we can access indirectly.

来源：https://stackoverflow.com/questions/31779507/what-is-riwo-read-indirectly-write-out-state

标签

java

static