Seven Major New Features
• Generics
• Autoboxing/Unboxing
• Enhanced for loop (“foreach”)
• Type-safe enumerations
• Varargs
• Static import
• Metadata
Autoboxing/Unboxing of Primitive Types
• Problem: (pre-J2SE 5.0)
> Conversion between primitive types and wrapper types (and vice-versa)
> You need manually convert a primitive type to a wrapper type before adding it to a collection
int i = 22;
List l = new LinkedList();
l.add(new Integer(i));
• Solution: Let the compiler do it
Byte byteObj = 22; // Autoboxing conversion
int i = byteObj // Unboxing conversion
ArrayList<Integer> al = new ArrayList<Integer>();
al.add(22); // Autoboxing conversion
Enhanced for Loop (foreach)
• Problem: (pre-J2SE 5.0)
> Iterating over collections is tricky
> Often, iterator only used to get an element
> Iterator is error prone
(Can occur three times in a for loop)
• Solution: Let the compiler do it
> New for loop syntax
for (variable : collection)
> Works for Collections and arrays
Enhanced for Loop Example
• Old code
void cancelAll(Collection c) {
for (Iterator i = c.iterator(); i.hasNext(); ){
TimerTask task = (TimerTask)i.next();
task.cancel();
}
}
• New Code
void cancelAll(Collection<TimerTask> c) {
for (TimerTask task : c)
task.cancel();
}
Type-safe Enumerations
• Problem: (pre-J2SE 5.0) Previously, if you wanted to define an enumeration you either:
> Defined a bunch of integer constants
> Followed one of the various “type-safe enum patterns”
• Issues of using Integer constants
> public static final int SEASON_WINTER = 0;
> Public static final int SEASON_SUMMER = 1;
> Not type safe (any integer will pass)
> No namespace (SEASON_*)
> Brittleness (how do add value in-between?)
> Printed values uninformative (prints just int values)
• Issues of using “type-safe enum patterns”
> Verbose
> Do not work well with switch statements
• Solution: New type of class declaration
> enum type has public, self-typed members for each enum constant
> New keyword, enum
Varargs
• Problem: (in pre-J2SE 5.0)
> To have a method that takes a variable number of parameters
> Can be done with an array, but caller has to create it first
> Look at java.text.MessageFormat
• Solution: Let the compiler do it for you
> public static String format
(String fmt, Object... args);
> Java now supports printf(...)
Varargs examples
• APIs have been modified so that methods accept variable-length argument lists where appropriate
> Class.getMethod
> Method.invoke
> Constructor.newInstance
> Proxy.getProxyClass
> MessageFormat.format
• New APIs do this too
> System.out.printf(“%d + %d = %d\n”, a, b, a+b);
Static Imports
• Problem: (pre-J2SE 5.0)
> Having to fully qualify every static referenced from external classes
• Solution: New import syntax
> import static TypeName.Identifier;
> import static Typename.*;
> Also works for static methods and enums
e.g Math.sin(x) becomes sin(x)
Formatted I/O
Simple Formatted I/O & Scanner
• Printf is popular with C/C++ developers
> Powerful, easy to use
• Finally adding printf to J2SE 5.0 (using varargs)
out.printf(“%-12s is %2d long”, name, l);
out.printf(“value = %2.2F”, value);
• Also a simple scanning API: convert text into primitives or Strings
Scanner s = new Scanner(System.in);
int n = s.nextInt();
Virtual Machine
Class Data Sharing
• Improved startup time
> especially for small applications
> up to 30% faster
• Reduced memory footprint
• During JRE installation, a set of classes are saved into a file, called a "shared archive"
• During subsequent JVM invocations, the shared archive is memory-mapped in
• -Xshare:on, -Xshare:off, -Xshare:auto, -Xshare:dump
Server Class Machine
• Auto-detected
> Application will use Java HotSpot Server VM
> Server VM starts slower but runs faster than Client VM
• 2 CPU, 2GB memory (except windows)
> Uses server compiler
> Uses parallel garbage collector
> Initial heap size is 1/64 of physical memory up to 1GB
> Max heap size is 1/4 of physical memory up to 1GB
JVM Self Tuning (Ergonomics)
• Maximum pause time goal
> -XX:MaxGCPauseMillis=<nnn>
> This is a hint, not a guarantee
> GC will adjust parameters to try and meet goal
> Can adversely effect application throughput
• Throughput goal
> -XX:GCTimeRatio=<nnn>
> GC Time : Application time = 1 / (1 + nnn)
> e.g. -XX:GCTimeRatio=19 (5% of time in GC)
Monitoring & Management
• Key component of RAS in the Java platform (Reliability, Availability, Serviceability)
• Features
> JVM instrumentation and integrated JMX
> Monitoring and management APIs
> Tools
JVM TI (JVM Tool Interface)
• New native programming interface for use by development and monitoring tools
> Replaces JVMPI (JVM Profiler Interface) and JVMDI (JVM Debugger Interface)
• Improved performance analysis
• Java Platform Debugger Architecture uses JVM TI and provides higher-level interface
• Supports bytecode level instrumentation
> Provides the ability to alter the Java virtual machine bytecode instructions which comprise the target program
J2SE 5.0 Monitoring & Management
Integrated JMX (JSR-003): MBean
• An MBean is a managed object that follows the design patterns conforming to the JMX specification
• An MBean can represent a device, an application, or any resource that needs to be managed
• The management interface of an MBean comprises a set of readable and/or writable attributes and a set of invokable operations
• MBeans can also emit notifications when predefined events occur
Platform Beans (MXBean's)
• Provides API access to
> number of classes loaded,
> threads running
> Thread state
> contention stats
> stack traces
> GC statistics
> memory consumption, low memory detection
> VM uptime, system properties, input arguments
> On-demand deadlock detection
JConsole
• JMX-compliant GUI tool that connects to a running JVM, which started with the management agent
• To start an application with the management agent for local monitoring, set the com.sun.management.jmxremote system property when you start the application
> JDK_HOME/bin/java -Dcom.sun.management.jmxremote
-jar JDK_HOME/demo/jfc/Java2D/Java2Demo.jar
• To start JConsole
> JDK_HOME/bin/jconsole
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