How does scala generated byte code drops the checked exception?

Question

If it possible to write byte code for a method that is supposed to throw a checked exception?

For instance the following Java class doesn't compile unless the method declares it throws the checked exception:

public class CheckedExceptionJava {
  public Class<?> testChecked(String s) throws ClassNotFoundException {
    return Class.forName(s);
  }
}

While the following Scala equivalent does ( because Scala doesn't have checked exceptions ) :

class CheckedException { 
    def testChecked( s : String )  = Class.forName( s ) 
}

Even if the bytecode generated are almost identical:

Compiled from "CheckedExceptionJava.java"
public class CheckedExceptionJava extends java.lang.Object{
public CheckedExceptionJava();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   return

public java.lang.Class testChecked(java.lang.String)   throws java.lang.ClassNotFoundException;
  Code:
   0:   aload_1
   1:   invokestatic    #2; //Method java/lang/Class.forName:(Ljava/lang/String;)Ljava/lang/Class;
   4:   areturn

}

Compiled from "CheckedException.scala"
public class CheckedException extends java.lang.Object implements scala.ScalaObject{
public CheckedException();
  Code:
   0:   aload_0
   1:   invokespecial   #24; //Method java/lang/Object."<init>":()V
   4:   return

public java.lang.Class testChecked(java.lang.String);
  Code:
   0:   aload_1
   1:   invokestatic    #11; //Method java/lang/Class.forName:(Ljava/lang/String;)Ljava/lang/Class;
   4:   areturn

}

Question: Is it possible ( and how ) to generate bytecode for that doesn't mark it throws a checked exception even if the code inside that method doesn't handle it?

score 14 · Accepted Answer · answered Dec 02 '10 at 16:31

Simple. While the JVM bytecode includes checked exception specifications on methods, the bytecode verifier that runs before any bytecode is executed specifically doesn't check that the methods actually conform to exception specifications. You could write a program that took existing JVM bytecode and removed all exception specifications, and the resulting bytecode would be perfectly valid and run identically to the original (barring reflection).

How does scala generated byte code drops the checked exception?

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