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_posts/2014-12-13-java-newfeature.md
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_posts/2014-12-13-java-newfeature.md
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---
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layout: post
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title: Java8 新特性
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tags: Java 新特性
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categories: Java
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---
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* TOC
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{:toc}
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# `1` 语言新特性
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## `1.1` Lambda
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自动推测形参类型`e`
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~~~ java
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Arrays.asList( "a", "b", "d" ).forEach( e -> System.out.println( e ) );
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~~~
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指定形参类型`e`
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~~~ java
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Arrays.asList( "a", "b", "d" ).forEach( ( String e ) -> System.out.println( e ) );
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~~~
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方法体可以用`{}`包裹
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~~~ java
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Arrays.asList( "a", "b", "d" ).forEach( e -> {
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System.out.print( e );
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System.out.print( e );
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} );
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~~~
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**effectively final**，lambda引用的对象会自动转为final
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~~~java
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String separator = ",";
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Arrays.asList( "a", "b", "d" ).forEach(
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( String e ) -> System.out.print( e + separator ) );
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~~~
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## `1.2` FunctionalInterface
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函数接口就是只具有一个方法的普通接口，这样的接口，可以被隐式转换为lambda表达式，
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然而，一旦在此接口中增加了方法，它将不再是函数接口，使用lambda时也将编译失败。
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`@FunctionalInterface`注解可以约束接口的行为，**默认方法**与**静态方法**不会影响函数接口
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~~~java
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@FunctionalInterface
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public interface Functional {
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void method();
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}
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~~~
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## `1.3` 接口默认方法
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~~~java
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public interface Defaulable {
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// Interfaces now allow default methods, the implementer may or
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// may not implement (override) them.
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default String notRequired() {
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return "Default implementation";
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}
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}
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public class DefaultableImpl implements Defaulable {
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}
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public class OverridableImpl implements Defaulable {
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@Override
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public String notRequired() {
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return "Overridden implementation";
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}
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}
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~~~
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## `1.4` 接口静态方法
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~~~java
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private interface DefaulableFactory {
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// Interfaces now allow static methods
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static Defaulable create( Supplier< Defaulable > supplier ) {
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return supplier.get();
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}
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}
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~~~
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## `1.5` 方法引用
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可以将类中既有 方法引用为lambda
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~~~java
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public static class Car {
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public static Car create( final Supplier< Car > supplier ) {
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return supplier.get();
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}
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public static void collide( final Car car ) {
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System.out.println( "Collided " + car.toString() );
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}
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public void repair() {
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System.out.println( "Repaired " + this.toString() );
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}
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public void follow( final Car another ) {
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System.out.println( "Following the " + another.toString() );
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}
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}
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~~~
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①构造器引用，语法为 `Class::new`，效果如同 `() -> new Class()`
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~~~java
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Car car = Car.create( Car::new );
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final List< Car > cars = Arrays.asList( car );
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~~~
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②静态方法引用，语法为 `Class::static_method`，效果如同 `p -> Class.static_method(p)`
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~~~java
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cars.forEach( Car::collide );
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~~~
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③实例**无参**方法引用，语法为 `Class::method`，效果如同 `p -> p.method()`，该方法没有参数
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~~~java
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cars.forEach( Car::repair );
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~~~
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④实例**有参**方法引用，语法为 `instance::method` ，效果如同 `p -> instance.method(p)`
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~~~java
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final Car police = Car.create( Car::new );
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cars.forEach( police::follow );
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~~~
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⑤其他
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~~~java
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super::methName //引用某个对象的父类方法
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TypeName[]::new //引用一个数组的构造器
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~~~
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## `1.6` 重复注解
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相同的注解可以在同一地方声明多次，由 `@Repeatable` 提供此特性
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~~~java
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/**
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* @Repeatable( Filters.class )
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* 表示该注解可重复使用，注解内容存放于Filters中
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*/
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@Target( ElementType.TYPE )
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@Retention( RetentionPolicy.RUNTIME )
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@Repeatable( Filters.class )
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public @interface Filter {
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String value();
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};
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/**
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* 存放@Filter注解的数组
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* 该注解必须可以被访问
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*/
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@Target( ElementType.TYPE )
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@Retention( RetentionPolicy.RUNTIME )
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public @interface Filters {
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Filter[] value();
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}
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/**
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* 测试
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*/
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@Filter( "filter1" )
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@Filter( "filter2" )
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public interface Filterable {
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}
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public static void main(String[] args) {
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//java8 提供的新方法，用于获取重复的注解
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for(Filter filter : Filterable.class.getAnnotationsByType(Filter.class ) ) {
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System.out.println( filter.value() );
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}
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}
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~~~
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## `1.7` 类型推测
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~~~java
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public class TypeInfer {
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public static void main(String[] args) {
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final Value< String > value = new Value<>();
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//value.setV(Value.<String>defV()); //以前的写法
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//Value.defV()返回类型被推测为String
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value.setV(Value.defV());
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System.out.println(value.getV());
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}
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}
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class Value< T > {
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private T o;
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public static <T> T defV() {
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//若T不为Object,将会出现类型转换异常
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return (T)new Object();
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}
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|
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public void setV(T o){
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this.o = o;
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}
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public T getV(){
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return o;
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}
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}
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~~~
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## `1.8` 扩展注解
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java8几乎可以为任何东西添加注解：局部变量、泛型类、父类与接口的实现以及方法异常
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`ElementType.TYPE_USE`和`ElementType.TYPE_PARAMETER` 用于描述注解上下文
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|
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~~~java
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public class AnnotationEX {
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@Retention( RetentionPolicy.RUNTIME )
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@Target( { ElementType.TYPE_USE, ElementType.TYPE_PARAMETER } )
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public @interface Anno {
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}
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|
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public static class TestAEX< @Anno T > extends @Anno Object {
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public void method() throws @Anno Exception {
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}
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}
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public static void main(String[] args) {
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final TestAEX< String > holder = new @Anno TestAEX<>();
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@Anno Collection< @Anno String > strings = new ArrayList<>();
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}
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}
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~~~
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# `2` 类库新特性
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## `2.1` Optional
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~~~java
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//将String对象装入Optional容器
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Optional< String > stringOptional = Optional.ofNullable( null );
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//判断容器中的String对象是否不为空
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System.out.println(stringOptional.isPresent() );
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//orElseGet通过lambda产生一个默认值
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System.out.println(stringOptional.orElseGet( () -> "[default string]" ) );
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//map对String对象进行转化，然后返回一个新的Optional实例，
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// 此处s为null，所以未转换直接返回了1个新Optional实例，
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// orElse直接产生一个默认值
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System.out.println(stringOptional.map( p -> "[" + p + "]" ).orElse( "Hello Optional" ) );
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~~~
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## `2.2` Stream
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stream操作被分成了**中间操作**与**最终操作**两种
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**中间操作**返回一个新的stream对象。中间操作总是采用惰性求值方式，运行一个像filter这样的中间操作实际上没有进行任何过滤，相反它在遍历元素时会产生了一个新的stream对象，这个新的stream对象包含原始stream中符合给定谓词的所有元素。
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**最终操作**可能直接遍历stream，产生一个结果或副作用，如forEach、sum等。当最终操作执行结束之后，stream管道被认为已经被消耗了，没有可能再被使用了。在大多数情况下，最终操作都是采用及早求值方式，及早完成底层数据源的遍历。
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~~~java
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public enum Status {
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OPEN, CLOSED
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};
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public class Task {
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private final Status status;
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private final Integer points;
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Task( final Status status, final Integer points ) {
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this.status = status;
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this.points = points;
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}
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public Integer getPoints() {
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return points;
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}
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public Status getStatus() {
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return status;
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}
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@Override
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public String toString() {
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return String.format( "[%s, %d]", status, points );
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}
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}
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|
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~~~
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|
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~~~java
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public static void main(String[] args){
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Collection< Task > tasks = Arrays.asList(
|
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new Task( Status.OPEN, 5 ),
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new Task( Status.OPEN, 13 ),
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new Task( Status.CLOSED, 8 )
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);
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|
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long totalPointsOfOpenTasks = tasks
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.stream()
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.filter( task -> task.getStatus() == Status.OPEN )
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.mapToInt( Task::getPoints )
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.sum();
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|
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System.out.println( "Total points: " + totalPointsOfOpenTasks );
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}
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~~~
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|
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原生并行处理
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|
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~~~java
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double totalPoints = tasks
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.stream()
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.parallel()
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.map( task -> task.getPoints() ) // or map( Task::getPoints )
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.reduce( 0, Integer::sum );
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|
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System.out.println( "Total points (all tasks): " + totalPoints );
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~~~
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分组
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|
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~~~java
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final Map< Status, List< Task >> map = tasks
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.stream()
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.collect( Collectors.groupingBy(Task::getStatus) );
|
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|
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System.out.println( map );
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~~~
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|
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权重
|
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|
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~~~java
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//计算权重
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final Collection< String > result = tasks
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.stream() // Stream< String >
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.mapToInt( Task::getPoints ) // IntStream
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.asLongStream() // LongStream
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.mapToDouble( points -> points / totalPoints ) // DoubleStream
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.boxed() // Stream< Double >
|
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.mapToLong( weigth -> ( long )( weigth * 100 ) ) // LongStream
|
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.mapToObj( percentage -> percentage + "%" ) // Stream< String>
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.collect( Collectors.toList() ); // List< String >
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|
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System.out.println( result );
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~~~
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## `2.3` Date/Time API
|
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|
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时间格式均为 **ISO-8601**
|
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|
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`Clock`
|
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|
||||
~~~java
|
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//日期时间都有，有时区
|
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Clock clock = Clock.systemUTC();
|
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System.out.println(clock.instant());
|
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System.out.println(clock.millis());
|
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~~~
|
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|
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`LocalDate`
|
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|
||||
~~~java
|
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//只有日期没有时间
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LocalDate localDate = LocalDate.now();
|
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LocalDate localDateFromClock = LocalDate.now(clock);
|
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System.out.println(localDate);
|
||||
System.out.println(localDateFromClock);
|
||||
~~~
|
||||
|
||||
`LocalTime`
|
||||
|
||||
~~~java
|
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//只有时间没有日期
|
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LocalTime localTime = LocalTime.now();
|
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LocalTime localTimeFromClock = LocalTime.now(clock);
|
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System.out.println(localTime);
|
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System.out.println(localTimeFromClock);
|
||||
~~~
|
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|
||||
`LocalDateTime `
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|
||||
~~~java
|
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//日期时间都有，没有时区
|
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LocalDateTime localDateTime = LocalDateTime.now();
|
||||
LocalDateTime localDateTimeFromClock = LocalDateTime.now(clock);
|
||||
System.out.println(localDateTime);
|
||||
System.out.println(localDateTimeFromClock);
|
||||
~~~
|
||||
|
||||
`ZonedDateTime`
|
||||
|
||||
~~~java
|
||||
//指定时区，只有ZonedDateTime，没有ZoneDate与ZoneTime
|
||||
ZonedDateTime zonedDateTime = ZonedDateTime.now();
|
||||
ZonedDateTime zonedDateTimeFromClock = ZonedDateTime.now();
|
||||
ZonedDateTime zonedDateTimeFromZone = ZonedDateTime.now(ZoneId.of( "America/Los_Angeles" ));
|
||||
System.out.println(zonedDateTime);
|
||||
System.out.println(zonedDateTimeFromClock);
|
||||
System.out.println(zonedDateTimeFromZone);
|
||||
~~~
|
||||
|
||||
`Duration`
|
||||
|
||||
~~~java
|
||||
//计算时间差
|
||||
LocalDateTime from = LocalDateTime.of( 2014, Month.APRIL, 16, 0, 0, 0 );
|
||||
LocalDateTime to = LocalDateTime.of( 2015, Month.APRIL, 16, 23, 59, 59 );
|
||||
Duration duration = Duration.between( from, to );
|
||||
System.out.println( "Duration in days: " + duration.toDays() );
|
||||
System.out.println( "Duration in hours: " + duration.toHours() );
|
||||
~~~
|
||||
|
||||
## `2.4` Nashorn
|
||||
|
||||
`javax.script.ScriptEngine`的另一种实现，允许js与java相互调用
|
||||
|
||||
~~~java
|
||||
ScriptEngineManager manager = new ScriptEngineManager();
|
||||
ScriptEngine engine = manager.getEngineByName( "JavaScript" );
|
||||
|
||||
System.out.println( engine.getClass().getName() );
|
||||
System.out.println( "Result:" + engine.eval( "function f() { return 1; }; f() + 1;" ) );
|
||||
~~~
|
||||
|
||||
## `2.5` Base64
|
||||
|
||||
Base64编码已经成为Java8类库的标准
|
||||
|
||||
~~~java
|
||||
final String text = "Base64 finally in Java 8!";
|
||||
|
||||
final String encoded = Base64
|
||||
.getEncoder()
|
||||
.encodeToString( text.getBytes(StandardCharsets.UTF_8 ) );
|
||||
System.out.println( encoded );
|
||||
|
||||
final String decoded = new String(
|
||||
Base64.getDecoder().decode( encoded ),
|
||||
StandardCharsets.UTF_8 );
|
||||
System.out.println( decoded );
|
||||
~~~
|
||||
|
||||
>其他编码器与解码器
|
||||
>
|
||||
>`Base64.getUrlEncoder() Base64.getUrlDecoder()`
|
||||
>
|
||||
>`Base64.getMimeEncoder() Base64.getMimeDecoder()`
|
||||
|
||||
## `2.6` 并行（parallel ）数组
|
||||
|
||||
**并行数组**操作可以在多核机器上极大提高性能
|
||||
|
||||
~~~java
|
||||
long[] arrayOfLong = new long [ 20000 ];
|
||||
|
||||
//对arrayLong所有元素随机赋值
|
||||
Arrays.parallelSetAll(arrayOfLong,
|
||||
index -> ThreadLocalRandom.current().nextInt(1000000));
|
||||
|
||||
//打印前10个元素
|
||||
Arrays.stream( arrayOfLong ).limit( 10 ).forEach(
|
||||
i -> System.out.print( i + " " ) );
|
||||
System.out.println();
|
||||
|
||||
//对arrayLong数组排序
|
||||
Arrays.parallelSort( arrayOfLong );
|
||||
|
||||
//打印前10个元素
|
||||
Arrays.stream( arrayOfLong ).limit( 10 ).forEach(
|
||||
i -> System.out.print( i + " " ) );
|
||||
System.out.println();
|
||||
~~~
|
||||
|
||||
## `2.7` 并发（concurrency）
|
||||
|
||||
* java.util.concurrent.`ConcurrentHashMap`类中加入了一些新方法来支持聚集操作
|
||||
|
||||
* java.util.concurrent.`ForkJoinPool`类中加入了一些新方法来支持共有资源池（common pool）
|
||||
|
||||
* java.util.concurrent.locks.`StampedLock`类提供基于容量的锁，这种锁有三个模型来控制读写操作
|
||||
|
||||
* java.util.concurrent.atomic包中增加新类：<br/>
|
||||
`DoubleAccumulator`，<br/>
|
||||
`DoubleAdder`，<br/>
|
||||
`LongAccumulator`，<br/>
|
||||
`LongAdder`
|
||||
|
||||
|
||||
# `3` 编译器新特性
|
||||
|
||||
## `3.1` 参数名
|
||||
|
||||
方法参数的名字保留在Java字节码中，并且能够在运行时获取它们
|
||||
|
||||
编译时需要加上 `–parameters` ，**maven-compiler-plugin** 可进行配置
|
||||
|
||||
~~~java
|
||||
public class ParameterNames {
|
||||
public static void main(String[] args) throws Exception {
|
||||
Method method = ParameterNames.class.getMethod( "main", String[].class );
|
||||
for( final Parameter parameter: method.getParameters() ) {
|
||||
System.out.println( "Parameter: " + parameter.getName() );
|
||||
}
|
||||
}
|
||||
}
|
||||
~~~
|
||||
|
||||
# `4` 新的Java工具
|
||||
|
||||
## `4.1` Nashorn引擎 `jjs`
|
||||
|
||||
它接受一些JavaScript源代码为参数，并且执行这些源代码
|
||||
|
||||
创建 **func.js** 文件
|
||||
|
||||
~~~javascript
|
||||
function f() {
|
||||
return 1;
|
||||
};
|
||||
|
||||
print( f() + 1 );
|
||||
~~~
|
||||
~~~bash
|
||||
jjs func.js
|
||||
~~~
|
||||
|
||||
## `4.2` 类依赖分析器 `jdeps`
|
||||
|
||||
它可以显示Java类的包级别或类级别的依赖，它接受一个.class文件，一个目录，或者一个jar文件作为输入。jdeps默认把结果输出到系统输出（控制台）上。
|
||||
|
||||
~~~bash
|
||||
jdeps org.springframework.core-3.0.5.RELEASE.jar
|
||||
~~~
|
||||
|
||||
如果依赖不在classpath中，就会显示 **not found**
|
||||
|
||||
# `5` JVM新特性
|
||||
|
||||
**PermGen**空间被移除了，取而代之的是**Metaspace**（JEP 122）。JVM选项**-XX:PermSize**与**-XX:MaxPermSize**分别被**-XX:MetaSpaceSize**与**-XX:MaxMetaspaceSize**所代替
|
||||
Reference in New Issue
Block a user