JVM/Java, are method accessibility rules enforced at runtime?

Question

I was curious about how the JVM works. Does the JVM acknowledge method accesibility rules like 'private' protected or is that only done at compile time?

For example, is it possible at around line37 to do some bytecode manipulation and call a protected method, say test3? Normally the compiler would not let me call that method because it is declared protected. But I was curious if that protected rule is enforced at runtime?

u.test1();
// Is it possible at runtime, to call 'test3' through bytecode manipulation
// @line37

package org.berlin.algo.basic.test;
public class RunX {
    private String zzz = "rrrrr";    
    public void test1() {
        // Note: I am intentionally use 'new' here as part of my test, not a
        // good practice I know but allowed by the language.
        Object x = new String("Test1 -----[1.1] " + zzz);
        x = new String("Test1 --- [1.2]" + x.toString());
        System.out.println(x);
        this.test2();
        this.test3();
    } 
    /**
     * Here, I noticed that the compiler removed my 'test2' code block.
     * Does that always happen?
     */
    private void test2() {
        Object x = new String("Test2@line21--->>> [2.1]");        
        System.out.println(x);
    }    
    protected void test3() {
        Object x = new String("Test3@line27 {Will the JVM enforce the 'protected' method rule for test3? --->>> [3.1]");
        x = new String("Test3@line28--->>> [3.2]");
        System.out.println(x);               
    }    
    public static void main(final String [] args) {
        System.out.println("Running");
        RunX u = new RunX();
        u.test1();
        // Is it possible at runtime, to call 'test3' through bytecode manipulation
        // @line37
        System.out.println("Done");
    }    
} // End of the Class //
/* 
 JVM bytecode: javap -v RunX 
 Compiled from "RunX.java"
public class org.berlin.algo.basic.test.RunX extends java.lang.Object
  SourceFile: "RunX.java"
  minor version: 0
  major version: 50
  Constant pool:
const #1 = class    #2; //  org/berlin/algo/basic/test/RunX
const #2 = Asciz    org/berlin/algo/basic/test/RunX;
...
...
const #84 = Asciz   SourceFile;
const #85 = Asciz   RunX.java;

{
public org.berlin.algo.basic.test.RunX();
  Code:
   Stack=2, Locals=1, Args_size=1
   0:   aload_0
   1:   invokespecial   #10; //Method java/lang/Object."<init>":()V
   4:   aload_0
   5:   ldc #12; //String rrrrr
   7:   putfield    #14; //Field zzz:Ljava/lang/String;
   10:  return
  LineNumberTable: 
   line 3: 0
   line 5: 4
   line 3: 10

  LocalVariableTable: 
   Start  Length  Slot  Name   Signature
   0      11      0    this       Lorg/berlin/algo/basic/test/RunX;

public void test1();
  Code:
   Stack=5, Locals=2, Args_size=1
   0:   new #21; //class java/lang/String
   3:   dup
   4:   new #23; //class java/lang/StringBuilder
   7:   dup
   8:   ldc #25; //String Test1 -----[1.1] 
   10:  invokespecial   #27; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V
   13:  aload_0
   14:  getfield    #14; //Field zzz:Ljava/lang/String;
   17:  invokevirtual   #30; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   20:  invokevirtual   #34; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   23:  invokespecial   #38; //Method java/lang/String."<init>":(Ljava/lang/String;)V
   26:  astore_1
   27:  new #21; //class java/lang/String
   30:  dup
   31:  new #23; //class java/lang/StringBuilder
   34:  dup
   35:  ldc #39; //String Test1 --- [1.2]
   37:  invokespecial   #27; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V
   40:  aload_1
   41:  invokevirtual   #41; //Method java/lang/Object.toString:()Ljava/lang/String;
   44:  invokevirtual   #30; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   47:  invokevirtual   #34; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   50:  invokespecial   #38; //Method java/lang/String."<init>":(Ljava/lang/String;)V
   53:  astore_1
   54:  getstatic   #42; //Field java/lang/System.out:Ljava/io/PrintStream;
   57:  aload_1
   58:  invokevirtual   #48; //Method java/io/PrintStream.println:(Ljava/lang/Object;)V
   61:  aload_0
   62:  invokespecial   #54; //Method test2:()V
   65:  aload_0
   66:  invokevirtual   #57; //Method test3:()V
   69:  return

  LocalVariableTable: 
   Start  Length  Slot  Name   Signature
   0      70      0    this       Lorg/berlin/algo/basic/test/RunX;
   27      43      1    x       Ljava/lang/Object;

protected void test3();
  Code:
   Stack=3, Locals=2, Args_size=1
   0:   new #21; //class java/lang/String
   3:   dup
   4:   ldc #66; //String Test3@line27 {Will the JVM enforce the 'protected' method rule for test3? --->>> [3.1]
   6:   invokespecial   #38; //Method java/lang/String."<init>":(Ljava/lang/String;)V
   9:   astore_1
   10:  new #21; //class java/lang/String
   13:  dup
   14:  ldc #68; //String Test3@line28--->>> [3.2]
   16:  invokespecial   #38; //Method java/lang/String."<init>":(Ljava/lang/String;)V
   19:  astore_1
   20:  getstatic   #42; //Field java/lang/System.out:Ljava/io/PrintStream;
   23:  aload_1
   24:  invokevirtual   #48; //Method java/io/PrintStream.println:(Ljava/lang/Object;)V
   27:  return

  LocalVariableTable: 
   Start  Length  Slot  Name   Signature
   0      28      0    this       Lorg/berlin/algo/basic/test/RunX;
   10      18      1    x       Ljava/lang/Object;
public static void main(java.lang.String[]);
  Code:
   Stack=2, Locals=2, Args_size=1
   0:   getstatic   #42; //Field java/lang/System.out:Ljava/io/PrintStream;
   3:   ldc #72; //String Running
   5:   invokevirtual   #74; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   8:   new #1; //class org/berlin/algo/basic/test/RunX
   11:  dup
   12:  invokespecial   #76; //Method "<init>":()V
   15:  astore_1
   16:  aload_1
   17:  invokevirtual   #77; //Method test1:()V
   20:  getstatic   #42; //Field java/lang/System.out:Ljava/io/PrintStream;
   23:  ldc #79; //String Done
   25:  invokevirtual   #74; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   28:  return
  LocalVariableTable: 
   Start  Length  Slot  Name   Signature
   0      29      0    args       [Ljava/lang/String;
   16      13      1    u       Lorg/berlin/algo/basic/test/RunX;
}
 */

Answer 1

To the JLS!

15.12.4 Runtime Evaluation of Method Invocation
At run time, method invocation requires five steps. First, a target reference may be computed. Second, the argument expressions are evaluated. Third, the accessibility of the method to be invoked is checked. Fourth, the actual code for the method to be executed is located. Fifth, a new activation frame is created, synchronization is performed if necessary, and control is transferred to the method code.

The wording of the JLS indicates that the accessibility would be checked at runtime.

Answer 2

The JVM does acknowledge these. They can be overridden, by calling setAccessible(true) as Prashant Bhate does, but by default they are enforced. (See http://download.oracle.com/javase/6/docs/api/java/lang/reflect/AccessibleObject.html#setAccessible%28boolean%29.)

By the way, you write that "the compiler doesn't encode type method visibility rules into the Java bytecde file"; but it does. In addition to the above, it has to encode these, for a number of reasons. For example:

• you can compile a class A that references class B even if you only have the compiled version of class B.
• you can inspect a method's visibility via reflection (the getModifiers() method).
• private methods aren't virtual -slash- can't be overridden by subclasses.