Bytecode Hacks

Bridger and Seven2Six

Presentation Prepared By:
David M. Lloyd <david.lloyd@redhat.com>

Bridger: The Problem

• You need to retain binary compatibility, but...
  • You have a method whose return type you want to narrow or broaden
  • You want to change the return type of a method to something completely different
    • For example, changing a void method to support call chaining
  • You want to remove a deprecated method

Bridger: The Solution

• Bridger executes a name transformation on all methods containing the string "$$bridge" in their name
• The "$$bridge" string (and everything after) is dropped
• The method is marked ACC_BRIDGE and ACC_SYNTHETIC
  • This means that javac won't see it
  • But code compiled against that signature will still link against it at runtime
• Thus all the aforementioned problems can be easily solved

Bridger: Example 1

    /**
     * This method returned Object in a previous version.
     */
    public String getValue() {
        return blah.toString();
    }

    /**
     * This gets changed to "getValue()" and is hidden from javac.
     * But, classes compiled against the old version will link against
     * this one instead at run time.
     */
    public Object getValue$$bridge() {
        return getValue();
    }

Bridger: Example 2

    /**
     * This method returned void in a previous version.
     */
    public Builder setName(String name) {
        this.name = name;
        return this;
    }

    /**
     * This gets changed to "setName()" and is hidden from javac.
     * But, classes compiled against the void-returning old version
     * will link against this one instead at run time.
     */
    public void setName$$bridge(String name) {
        this.name = name;
    }

Seven2Six: The Problem

• Someone gave you a PR riddled with diamond operators but you need to backport it to your Java 6 branch
• You need to support Java 6 but you really want String switch, try-with-resources, etc.

Seven2Six: The Solution

• Translates Java 7 bytecode to Java 6
• Fully supports:
  • Diamond operator
  • String switch
  • Improved type inference
  • New numeric literals (binary constants, underscore separators)
• Mostly supports:
  • Try-with-resources (suppressed exceptions are dropped when running on Java 6, but work as expected on Java 7)
  • Multi-catch blocks (with one caveat)
    • If the compiler detects a common superclass of more than one of the exception options, it will silently link against it
    • If that class is missing when running Java 6, you will get errors at run time
• Does not support:
  • The invokedynamic instruction
  • Any JDK 7-specific APIs

Get this stuff

• Get bridger from GitHub or Maven as a plugin (org.jboss.bridger:bridger:1.0.Final)
• Get seven2six from GitHub or Maven as a plugin (org.jboss.seven2six:seven2six:1.1.Final)

That's it!