Wrap around pattern is not listed in in GOF book but is very useful for problem like below:

• Loop construct for e.g do while/while/for loop
• Stopwatch around some code.
• Wrap checked exception with run time exception
• Initialization and cleanup for eg Threadpool creation/destruction or file open/close etc
• Adding context info to threads for eg request context info for logging or passing security context etc

Lot of plumbing code is required in java to do such simple things. Java8 added lamdba support and that has answer for such problems.

Using Lambda behavior can be passed as argument to any function and it is very powerful thing if you want to solve above problems.

Wrap Around

Template of wrap around function is something like below

• Pre Code
• Actual behavior
• Post Code

WrapAround for loop

@FunctionalInterface
 public interface CodeBlock {
 void execute();
 }

 @FunctionalInterface
 public interface Condition {
 boolean test();
 }

 public static void loop(Condition condition, CodeBlock codeBlock) {
 while (condition.test()) {
 codeBlock.execute();
 }
 }

Above code is very straight forward , it has 2 functional interface one for condition & another one for block of code to execute and those 2 behavior is passed to loop function using lambda.

This allows us to introduce new construct.

Lets look at some more example

WrapAround for time/stopwatch

@FunctionalInterface
 public interface CodeBlock {
 void execute();
 }
 
 public static void time(String name, CodeBlock codeBlock) {
 long start = System.currentTimeMillis();
 codeBlock.execute();
 long total = System.currentTimeMillis() - start;
 System.out.println(name + " took " + total + " ms");
 }

WrapAround Closable/Exception

@FunctionalInterface
 public interface AutoCodeBlock {
 void execute(AutoCloseable closeable) throws IOException;
 }

 @FunctionalInterface
 public interface ExceptionBlock {
 void execute() throws Exception;
 }
 
 public static void withAutoClose(AutoCloseable resource, AutoCodeBlock codeBlock) throws Exception {
 try (AutoCloseable c = resource) {
 codeBlock.execute(c);
 }
 }

 public static void wrapWithRuntimeException(ExceptionBlock codeBlock) {
 try {
 codeBlock.execute();
 } catch (Exception e) {
 throw new RuntimeException(e);
 }
 }

Java 8 has tons of feature that can make code concise and i used just one the the feature implement really useful thing.

• Code used in blog is available @ github