Writing a detector to search for uses of "System.out.println" using Findbugs

Question

I am trying to write a bug detector to find instances of the method call "System.out.println" using Findbugs.

I understand that "System.out.println" in bytecode is compiled to a call to GETSTATIC, which pushes "System.out" onto the stack. A call to INVOKEVIRTUAL pops "System.out" off the stack and calls the method.

I have prepared some code (found below) which finds the correct GETSTATIC and INVOKEVIRTUAL calls, but have been unable to link the two together. I suspect I may need to use OpcodeStack in some way, but am having trouble in understanding how I can use it. Any help would be appreciated.

    @Override 
    public void sawOpcode(int seen) { 
            // if opcode is getstatic 
            if (seen == GETSTATIC) { 
                    String clsName = getClassConstantOperand(); 
                    if ("java/lang/System".equals(clsName)) { 
                            String fldName = getNameConstantOperand(); 
                            if ("out".equals(fldName)) { 
                                    System.out.println("SYSTEM.OUT here"); 
                            } 
                    } 
            } 

            // if opcode is invokevirtual 
            if (seen == INVOKEVIRTUAL) { 
                    String cls = getDottedClassConstantOperand(); 
                    if ("java.io.PrintStream".equals(cls)) { 
                            String methodName = getNameConstantOperand(); 
                            if ("println".equals(methodName)) { 
                                    bugReporter.reportBug(new BugInstance("SYSTEM_OUT_PRINTLN", 
                                                    NORMAL_PRIORITY).addClassAndMethod(this) 
                                                    .addSourceLine(this)); 
                            } 
                    } 
            } 

    }

Answer 1

I found that, for my use case, it is enough to determine that System.out or System.err are used at all - in 99% of cases, these will be used for calling .print or .println later in the block. My detector detects GET_STATIC opcodes that load System.err or System.out. The code is below, showing 3 alternatives of determining that this occurs.

package my.findbugs.detectors.forbiddencalls;

import org.apache.log4j.Logger; // it is not trivial to use a logger with FindBugs in Eclipse, leave it out if there are problems

import my.findbugs.detectors.util.DetectorUtil;

import edu.umd.cs.findbugs.BugInstance;
import edu.umd.cs.findbugs.BugReporter;
import edu.umd.cs.findbugs.bcel.OpcodeStackDetector;
import edu.umd.cs.findbugs.classfile.ClassDescriptor;
import edu.umd.cs.findbugs.classfile.FieldDescriptor;


public class CallToSystemOutPrintlnDetector2 extends OpcodeStackDetector {

    private static final Logger LOGGER = Logger.getLogger(CallToSystemOutPrintlnDetector2.class);
    private BugReporter bugReporter;


    public CallToSystemOutPrintlnDetector2(BugReporter bugReporter) {
        super();
        this.bugReporter = bugReporter;
        LOGGER.debug("Instantiated.");
    }


    public void sawOpcode(int seen) {

        // find occurrences of:  
        //2:   getstatic       #54; //Field java/lang/System.out:Ljava/io/PrintStream;
//2:   getstatic       #54; //Field java/lang/System.out:Ljava/io/PrintStream;

        if (seen == GETSTATIC){

            try {
//              LOGGER.debug(operand); // static java.lang.System.out Ljava/io/PrintStream;
//              LOGGER.debug(operand.getClass()); // class edu.umd.cs.findbugs.classfile.analysis.FieldInfo
//              LOGGER.debug(operand.getName()); // err
//              LOGGER.debug(operand.getClassDescriptor()); // java/lang/System
//              LOGGER.debug(operand.getSignature()); // Ljava/io/PrintStream;

                FieldDescriptor operand = getFieldDescriptorOperand();
                ClassDescriptor classDescriptor = operand.getClassDescriptor();
                if ("java/lang/System".equals(classDescriptor.getClassName()) && 
                        ("err".equals(operand.getName())||"out".equals(operand.getName()))) {
                    reportBug();
                }
            } catch (Exception e) {
                //ignore
            }

            // could be used
//          try {
//              MethodDescriptor operand = getMethodDescriptorOperand();
//              LOGGER.debug(operand); // java.lang.System.outLjava/io/PrintStream;
//              LOGGER.debug(operand.getClass()); // class edu.umd.cs.findbugs.classfile.MethodDescriptor
//              LOGGER.debug(operand.getName()); // err 
//              LOGGER.debug(operand.getClassDescriptor()); // java/lang/System
//              LOGGER.debug(operand.getSignature()); // Ljava/io/PrintStream;
//          } catch (Exception e) {
//              //ignore
//          }

            // could be used
//          try {
//              String operand = getRefConstantOperand();
//              LOGGER.debug(operand); // java.lang.System.out : Ljava.io.PrintStream;
//              if (operand != null && (
//                  operand.startsWith("java.lang.System.out :") || operand.startsWith("java.lang.System.err :"))) {
//                  reportBug();
//              }
//          } catch (Exception e) {
//              //ignore
//          }
        }
    }

    private void reportBug(){
        this.bugReporter.reportBug(getBugInstance());
    }


    private BugInstance getBugInstance() {
        return new BugInstance(this, "MY_CALL_TO_SYSTEM_OUT_BUG", DetectorUtil.MY_PRIORITY)
            .addClassAndMethod(this)
            .addSourceLine(this);
    }

}

Answer 2

Your task is a bit more complicated than it seems. A simple case:

System.out.println("abc");

Is translated into a simple bytecode as well:

getstatic   #2; //java/lang/System.out
ldc #3; //String abc
invokevirtual   #4; //Calling java/io/PrintStream.println(String)

---

However if you are trying to print anything except simple constant/known value it gets harder:

int x = 42;
System.out.println(x + 17);

Will be translated to:

bipush  42
istore_1  //x = 42
getstatic   #2; //java/lang/System.out
iload_1  //x
bipush  17
iadd  //x + 17 on the stack
invokevirtual   #5; //Calling java/io/PrintStream.println(int)

---

But wait, it can get worse:

System.out.println("x times 27 is " + x * 27);

What? StringBuilder: ?

new #6; //new java/lang/StringBuilder()
dup
invokespecial   #7; //Calling java/lang/StringBuilder()
ldc #8; //String x times 2 is
invokevirtual   #9; //Calling java/lang/StringBuilder.append(String)
iload_1  //x
bipush  27
imul  //x * 27 on the stack
invokevirtual   #10; //Calling java/lang/StringBuilder.append:(int) with 'x * 27' argument
invokevirtual   #11; //Calling java/lang/StringBuilder.toString:()
invokevirtual   #4; //Calling java/io/PrintStream.println(String)

Interestingly, the original code was translated to (which is a known Java 5 (?) optimization):

System.out.println(
  new StringBuilder().
    append("x times 27 is ").
    append(x * 27).
    toString()
  );

---

Solution

So indeed - you will need a stack and you'll have to keep track of every push/pop operation as defined in bytecode instruction. A lot of work for such a simple task...

But if you go this path, solving the problem is quite simple: when you encounter INVOKEVIRTUAL the top of the stack should contain some value and the value below the top should be a "java/lang/System.out".

That being said I'm 100% sure Findbugs already implemented this and probably you can use some FindBugs API to make your life easier.