Excess Null Checker

Description

A tool that searches .java or .class files to detect excess null checks like the following:

Object x = null;
if (x == null) { // Condition is always true
    System.out.println("x is null");
}

System.out.println(x.hashCode());
if (x == null) { // Condition is always false
    System.out.println("x == null");
}

gradle run --args='<fileNameOrDirectory>'

<fileNameOrDirectory> should be

.class file
.java. In this case, javac tool is used to obtain .class files
<fileNameOrDirectory> (limited functionality). In this case, all .class and .java files are analyzed within the passed directory.

gradle test

Test files are located in app/src/test/resources directory.

Expected warnings placed directly to test files and marked with the following markers:

// Test: (condition_is_always_true|condition_is_always_false)

The tester uses them to identify if the tool correctly reports expected warnings and does not report false messages.

Full workflow located in Analyzer

In the beginning, it collects declarations (fields) for further use (DeclCollector).
Secondly, it builds Control-flow graph that is used to detect points where the state should be forked or merged. These points are linked with opcode instructions and being created on conditional and unconditional jumps.
After CFG nodes are initialized, it bypasses methods in the following order and tries to find excess null checks:
1. Static constructor
2. Ordinary constructors
3. Ordinary methods
It interprets every instruction and changes the state of the current stack. On every joint point obtained from CFG it forks or merges corresponding states. On every IFNULL or IFNONNULL it analyzes the DataEntry on the top of the stack and reports corresponding warning if it is determined (Null or NotNull)

The state contains the stack that stores a set of local variables and fields represented by DataEntry. DataEntry has the following properties:

name. It can be <Uninitialized>, <Dirty>, another string for a field, and an integer value for a local variable.
type. It can be Uninitialized, Other, Null, NotNull.

During the merging the following considerations are used:

Also, some optimization is used during resolving CFG nodes (see tests).