Flow类

JLS参考：https://docs.oracle.com/javase/specs/jls/se7/html/jls-16.html

This pass implements dataflow analysis for Java programs.

1、Liveness analysis checks that every statement is reachable.

2、Exception analysis ensures that every checked exception that is thrown is declared or caught.

3、Definite（一定的、确定的） assignment analysis ensures that each variable is assigned when used.

eg1:

任何局部变量在使用前要进行初始化，包括基本类型，如下将报错：

int i;
if(i==2){ // 报错，局部变量i未初始化
 
}

Definite unassignment analysis ensures that no final variable is assigned more than once（不止一次）.

eg2:

final int i;
i = 1;
i = 22; // The final local variable i may already have been assigned

The JLS has a number of problems in the specification of these flow analysis problems.This implementation attempts to address those issues.

JLS对数据流分析还存在几个问题，下面试着解决这些问题。

First, there is no accommodation for a finally clause that cannot complete normally.

1、For liveness analysis, an intervening finally clause can cause a break, continue, or return not to reach its target.

eg3:

for(int i=0;i<2;i++){
	System.out.println(i);
	try{
		return;
	}finally{
		continue;
	}
}

输出的结果为：0 1。可以看到finally中的continue让return没有达到它使用的目的。　　

2、For exception analysis, an intervening finally clause can cause any exception to be "caught".

eg1:

什么是intervening finally，就比如finally中有return 、continue等有控制流程转移的语句。

for(int i=0;i<2;i++){
			try{
				System.out.println("a");
				break;
			}catch(Exception ex){
 
			}finally{ // finally block does not complete normally
				System.out.println("b");
				continue;
			}
}　　

输出为：a b a b

@SuppressWarnings("finally")
private boolean isReturnWithinFinally() {
		try {
			throw new IOException();
		} finally {
			return true; // This hides the exception
		}
}

java里面的异常分为可不获和不可捕获两类，即便使用到catch块，也会导致非捕获的错误被finally吃掉。因此，return一定要放到finally外面。

关于intervening finally 参考文章：https://www.cnblogs.com/extjs4/p/9375400.html

3、For DA/DU analysis, the finally clause can prevent a transfer of control（控制权的转移） from propagating（传播） DA/DU
state to the target. In addition（另外）, code in the finally clause can affect the DA/DU status of variables.

4、For try statements, we introduce the idea of a variable being definitely unassigned "everywhere" in a block.
A variable V is "unassigned everywhere" in a block iff（if and only if） it is unassigned at the beginning of the block and there is no reachable assignment to V in the block.

An assignment V=e is reachable iff V is not DA after e. Then we can say that V is DU at the beginning of the catch block iff V is DU everywhere in the try block.

Similarly,V is DU at the beginning of the finally block iff V is DU everywhere in the try block and in every catch block.

说明：try statement的文法结构如下：

TryStatement:
    try Block Catches
    try Block Catchesopt Finally
    TryWithResourcesStatement

Specifically, the following bullet is added to 16.2.2

       V is unassigned everywhere in a block if it is
       unassigned before the block and there is no reachable
       assignment to V within the block.   

In 16.2.15, the third bullet (and all of its sub-bullets) for all try blocks is changed to

       V is definitely unassigned before a catch block iff V is
       definitely unassigned everywhere in the try block.
   

The last bullet (and all of its sub-bullets) for try blocks that have a finally block is changed to

       V is definitely unassigned before the finally block iff
       V is definitely unassigned everywhere in the try block
       and everywhere in each catch block of the try statement.   

In addition,

       V is definitely assigned at the end of a constructor iff
       V is definitely assigned after the block that is the body
       of the constructor and V is definitely assigned at every
       return that can return from the constructor.

In addition, each continue statement with the loop as its target is treated as a jump to the end of the loop body,and "intervening" finally clauses are treated as follows:

V is DA "due to（归因于） the continue" iff V is DA before the continue statement or V is DA at the end of any intervening finally block.

V is DU "due to the continue" iff any intervening finally cannot complete normally or V is DU at the end of every intervening finally block. This "due to the continue" concept is then used in the spec for the loops.

Similarly, break statements must consider intervening finally blocks.

For liveness analysis, a break statement for which any intervening finally cannot complete normally is not considered to cause the target statement to be
able to complete normally. Then we say V is DA "due to the break" iff V is DA before the break or V is DA
at the end of any intervening finally block. V is DU "due to the break" iff any intervening finally cannot complete normally
or V is DU at the break and at the end of every intervening finally block. (I suspect this latter condition can be simplified.)
This "due to the break" is then used in the spec for all statements that can be "broken".

The return statement is treated similarly. V is DA "due to a return statement" iff V is DA before the return statement
or V is DA at the end of any intervening finally block. Note that we don't have to worry about the return expression because this concept is only used for construcrors.

There is no spec in JLS2 for when a variable is definitely assigned at the end of a constructor, which is needed for
final fields (8.3.1.2). We implement the rule that V is DA at the end of the constructor iff it is DA and the end of
the body of the constructor and V is DA "due to" every return of the constructor.

Intervening finally blocks similarly affect exception analysis. An intervening finally that cannot complete normally allows us to ignore an otherwise uncaught exception.

To implement the semantics of intervening finally clauses, all nonlocal transfers (break, continue, return, throw,
method call that can throw a checked exception, and a constructor invocation that can thrown a checked exception)
are recorded in a queue, and removed from the queue when we complete processing the target of the nonlocal transfer.
This allows us to modify the queue in accordance with the above rules when we encounter a finally clause.
The only exception to this [no pun intended] is that checked exceptions that are known to be caught or declared
to be caught in the enclosing method are not recorded in the queue, but instead are recorded in a global variable
"Set thrown" that records the type of all exceptions that can be thrown.

Other minor issues the treatment of members of other classes (always considered DA except that within an anonymous class constructor, where DA status from the enclosing scope is preserved),

treatment of the case expression (V is DA before the case expression iff V is DA after the switch expression), treatment of variables declared in a
switch block (the implied DA/DU status after the switch expression is DU and not DA for variables defined in a switch block),
the treatment of boolean ?: expressions (The JLS rules only handle b and c non-boolean; the new rule is that
if b and c are boolean valued, then V is (un)assigned after a?b:c when true/false iff V is (un)assigned after b
when true/false and V is (un)assigned after c when true/false).

There is the remaining question of what syntactic forms constitute（组成） a reference to a variable. It is conventional to
allow this.x on the left-hand-side to initialize a final instance field named x, yet this.x isn't considered a "use" when appearing on a right-hand-side in most implementations.

Should parentheses affect what is considered a variable reference?（括号会影响被视为变量引用的内容）The simplest rule would be to allow unqualified forms only, parentheses optional, and phase out support for assigning to a final field via this.x.