Java 13的新增功能

「爱情、让人受尽委屈。」 2023-02-15 05:05 4阅读 0赞

![Java徽标][Java]![Java徽标][Java]

[Java 13][]已于2019年9月17日正式发布， [可在此处下载Java 13][Java 13 1]或此[openJDK已归档][openJDK] 。

Java 13功能。

*  [JEP 350：动态CDS存档][JEP 350_CDS]
 *  [JEP 351：ZGC：取消提交未使用的内存][JEP 351_ZGC]
 *  [JEP 353：重新实现旧版套接字API][JEP 353_API]
 *  [JEP 354：开关表达式（预览）][JEP 354] （开发人员功能）
 *  [JEP 355：文本块（预览）][JEP 355] （开发人员功能）

## [JEP 350动态CDS档案][JEP 350_CDS] ##

Java 10引入了[JEP 310 Application Class-Data Sharing][] 。 该JEP简化了创建CDS存档的过程。

此命令创建`.jar`的CDS存档文件。

$ java -XX:ArchiveClassesAtExit=hello.jsa -cp hello.jar Hello

此命令使用现有的CDS归档文件运行`.jar` 。

$  bin/java -XX:SharedArchiveFile=hello.jsa -cp hello.jar Hello

类数据共享（CDS）通过一次创建一个类数据存档并重新使用它来提高启动性能，这样JVM无需再次创建它。

请阅读以下文章，以了解有关CDS的更多信息：

*  [cl4cds][]
 *  [通过应用程序类数据共享缩短Java 13的启动时间][Java 13 2]

## [JEP 351 ZGC：取消提交未使用的内存][JEP 351_ZGC] ##

Java 11引入了[JEP 333：Z垃圾收集器（实验性）][JEP 333_Z] ； 在清理堆内存时，它提供了短暂的暂停时间。 但是，即使长时间未使用，它也不会将未使用的堆内存返回给操作系统。

该JEP通过将未使用的堆内存返回给操作系统来增强ZGC。

## [JEP-353重新实现旧版套接字API][JEP 353_API] ##

`java.net.Socket`和`java.net.ServerSocket`的基础实现是古老的，可以追溯到JDK 1.0，JDK 1.0是难以维护和调试的遗留Java和C代码的组合。 该JEP为Socket API引入了新的基础实现，这是Java 13中的默认实现。

在Java 13之前，它将`PlainSocketImpl`用于`SocketImpl`

ServerSocket.java

public class ServerSocket implements java.io.Closeable {
    
        /**
         * The implementation of this Socket.
         */
        private SocketImpl impl;
    
    }

Java 13引入了新的`NioSocketImpl`类，以代替`PlainSocketImpl` 。 但是，如果出现问题，我们仍然可以通过设置`jdk.net.usePlainSocketImpl`系统属性来切换回旧的实现`PlainSocketImpl` 。

查看一个简单的Socket示例。

JEP353.java

import java.io.IOException;
    import java.net.ServerSocket;
    import java.net.Socket;
    
    public class JEP353 {
    
        public static void main(String[] args) {
    
            try (ServerSocket serverSocket = new ServerSocket(8888)){
    
                boolean running = true;
                while(running){
    
                    Socket clientSocket = serverSocket.accept();
                    //do something with clientSocket
                }
    
            } catch (IOException e) {
                e.printStackTrace();
            }
    
        }
    }

在Java 13中，默认实现是`NioSocketImpl`

Terminal

D:\test>javac JEP353.java
    
    D:\test>java JEP353
    
    D:\test>java -XX:+TraceClassLoading JEP353  | findStr Socket
    
    [0.040s][info   ][class,load] java.net.ServerSocket source: jrt:/java.base
    [0.040s][info   ][class,load] jdk.internal.access.JavaNetSocketAccess source: jrt:/java.base
    [0.040s][info   ][class,load] java.net.ServerSocket$1 source: jrt:/java.base
    [0.040s][info   ][class,load] java.net.SocketOptions source: jrt:/java.base
    [0.040s][info   ][class,load] java.net.SocketImpl source: jrt:/java.base
    [0.044s][info   ][class,load] java.net.SocketImpl$$Lambda$1/0x0000000800ba0840 source: java.net.SocketImpl
    [0.047s][info   ][class,load] sun.net.PlatformSocketImpl source: jrt:/java.base
    
    [0.047s][info   ][class,load] sun.nio.ch.NioSocketImpl source: jrt:/java.base
    
    [0.047s][info   ][class,load] sun.nio.ch.SocketDispatcher source: jrt:/java.base
    [0.052s][info   ][class,load] java.net.SocketAddress source: jrt:/java.base
    [0.052s][info   ][class,load] java.net.InetSocketAddress source: jrt:/java.base
    [0.052s][info   ][class,load] java.net.InetSocketAddress$InetSocketAddressHolder source: jrt:/java.base
    [0.053s][info   ][class,load] sun.net.ext.ExtendedSocketOptions source: jrt:/java.base
    [0.053s][info   ][class,load] jdk.net.ExtendedSocketOptions source: jrt:/jdk.net
    [0.053s][info   ][class,load] java.net.SocketOption source: jrt:/java.base
    [0.053s][info   ][class,load] jdk.net.ExtendedSocketOptions$ExtSocketOption source: jrt:/jdk.net
    [0.053s][info   ][class,load] jdk.net.SocketFlow source: jrt:/jdk.net
    [0.053s][info   ][class,load] jdk.net.ExtendedSocketOptions$PlatformSocketOptions source: jrt:/jdk.net
    [0.053s][info   ][class,load] jdk.net.ExtendedSocketOptions$PlatformSocketOptions$1 source: jrt:/jdk.net
    [0.054s][info   ][class,load] jdk.net.ExtendedSocketOptions$1 source: jrt:/jdk.net
    [0.054s][info   ][class,load] sun.nio.ch.NioSocketImpl$FileDescriptorCloser source: jrt:/java.base
    [0.055s][info   ][class,load] java.net.Socket source: jrt:/java.base

我们可以通过设置`Djdk.net.usePlainSocketImpl`系统属性来切换回`PlainSocketImpl` 。

Terminal

D:\test>java -Djdk.net.usePlainSocketImpl -XX:+TraceClassLoading JEP353  | findStr Socket
    
    [0.041s][info   ][class,load] java.net.ServerSocket source: jrt:/java.base
    [0.041s][info   ][class,load] jdk.internal.access.JavaNetSocketAccess source: jrt:/java.base
    [0.041s][info   ][class,load] java.net.ServerSocket$1 source: jrt:/java.base
    [0.041s][info   ][class,load] java.net.SocketOptions source: jrt:/java.base
    [0.041s][info   ][class,load] java.net.SocketImpl source: jrt:/java.base
    [0.045s][info   ][class,load] java.net.SocketImpl$$Lambda$1/0x0000000800ba0840 source: java.net.SocketImpl
    [0.048s][info   ][class,load] sun.net.PlatformSocketImpl source: jrt:/java.base
    [0.048s][info   ][class,load] java.net.AbstractPlainSocketImpl source: jrt:/java.base
    
    [0.048s][info   ][class,load] java.net.PlainSocketImpl source: jrt:/java.base
    
    [0.048s][info   ][class,load] java.net.AbstractPlainSocketImpl$1 source: jrt:/java.base
    [0.050s][info   ][class,load] sun.net.ext.ExtendedSocketOptions source: jrt:/java.base
    [0.050s][info   ][class,load] jdk.net.ExtendedSocketOptions source: jrt:/jdk.net
    [0.050s][info   ][class,load] java.net.SocketOption source: jrt:/java.base
    [0.051s][info   ][class,load] jdk.net.ExtendedSocketOptions$ExtSocketOption source: jrt:/jdk.net
    [0.051s][info   ][class,load] jdk.net.SocketFlow source: jrt:/jdk.net
    [0.051s][info   ][class,load] jdk.net.ExtendedSocketOptions$PlatformSocketOptions source: jrt:/jdk.net
    [0.051s][info   ][class,load] jdk.net.ExtendedSocketOptions$PlatformSocketOptions$1 source: jrt:/jdk.net
    [0.051s][info   ][class,load] jdk.net.ExtendedSocketOptions$1 source: jrt:/jdk.net
    [0.051s][info   ][class,load] java.net.StandardSocketOptions source: jrt:/java.base
    [0.051s][info   ][class,load] java.net.StandardSocketOptions$StdSocketOption source: jrt:/java.base
    [0.053s][info   ][class,load] sun.net.ext.ExtendedSocketOptions$$Lambda$2/0x0000000800ba1040 source: sun.net.ext.ExtendedSocketOptions
    [0.056s][info   ][class,load] java.net.SocketAddress source: jrt:/java.base
    [0.056s][info   ][class,load] java.net.InetSocketAddress source: jrt:/java.base
    [0.058s][info   ][class,load] java.net.InetSocketAddress$InetSocketAddressHolder source: jrt:/java.base
    [0.059s][info   ][class,load] java.net.SocketCleanable source: jrt:/java.base

## [JEP-354开关表达式（预览）][JEP 354] ##

Java 12引入了[JEP 325 Switch表达式][JEP 325 Switch] 。 该JEP放弃了`break value` ，转而使用`yield`关键字以从`switch`表达式返回一个值。

*PS这是Java 13中的预览语言功能*

在传统的`switch`语句中，我们可以返回这样的值：

private static String getNumber(int number) {
            String result = "";
            switch (number) {
                case 1:
                case 2:
                    result = "one or two";
                    break;
                case 3:
                    result = "three";
                    break;
                case 4:
                case 5:
                case 6:
                    result = "four or five or six";
                    break;
                default:
                    result = "unknown";
            }
            ;
            return result;
        }

在Java 12中，我们可以使用`break`从`switch`返回一个值。

private static String getNumberViaBreak(int number) {
          String result = switch (number) {
              case 1, 2:
                  break "one or two";
              case 3:
                  break "three";
              case 4, 5, 6:
                  break "four or five or six";
              default:
                  break "unknown";
          };
          return result;
      }

在Java 13中，放弃了上述Java 12 `value break` ，而使用`yield`关键字来返回值。

private static String getNumberViaYield(int number) {
          return switch (number) {
              case 1, 2:
                  yield "one or two";
              case 3:
                  yield "three";
              case 4, 5, 6:
                  yield "four or five or six";
              default:
                  yield "unknown";
          };
      }

或像这样

private static String getNumberViaYield2(int number) {
          return switch (number) {
              case 1, 2:
                  yield "one or two";
              case 3:
                  yield "three";
              case 4, 5, 6:
                  int i = 0;
                  i++;
                  yield "four or five or six : " + i;
              default:
                  yield "unknown";
          };
        }

Java 13仍支持规则标签或箭头或`case L`

private static String getNumberViaCaseL(int number) {
          return switch (number) {
              case 1, 2 -> "one or two";
              case 3 -> "three";
              case 4, 5, 6 -> "four or five or six";
              default -> "unknown";
          };
      }

或者像这样，混合使用箭头语法和`yield` 。

private static String getNumberViaCaseL2(int number) {
          return switch (number) {
              case 1, 2 -> "one or two";
              case 3 -> "three";
              case 4, 5, 6 -> {
                  int i = 0;
                  i++;
                  yield "four or five or six :" + 1;
              }
              default -> "unknown";
          };
      }

> **Java 14**  
> 该开关表达式成为Java 14 – [JEP 361][]的标准功能。

> **注意**  
> 有关完整示例，请阅读此[Java 13 Switch Expressions。][Java 13 Switch Expressions]

## [JEP-355文本块（预览）][JEP 355] ##

该JEP最终引入了多行字符串文字，文本块。

*PS这是Java 13中的预览语言功能*

Java 13之前

String html ="<html>\n" +
    			  "   <body>\n" +
    			  "      <p>Hello, World</p>\n" +
    			  "   </body>\n" +
    			  "</html>\n";
    
    
     String json ="{\n" +
    			  "   \"name\":\"mkyong\",\n" +
    			  "   \"age\":38\n" +
    			  "}\n";

现在Java 13

String html =  """
                    <html>
                        <body>
                            <p>Hello, World</p>
                        </body>
                    </html>
    				        """;
    
     String json = """
                    {
                        "name":"mkyong",
                        "age":38
                    }
                    """;

**进一步阅读**  
更多多行字符串，文本块的示例

> **注意**  
> 此文本块在[Java 14中][Java 14]具有第二个预览[– JEP 368][Java 14] ，添加了两个新的转义序列：
> 
>  *  `\<end-of-line>`禁止行终止。
>  *  `\s`被翻译成一个空格。

要启用Java 13预览功能：

javac --enable-preview --release 13 Example.java
    java --enable-preview Example

## 下载源代码 ##

> $ git clone [https://github.com/mkyong/core-java][https_github.com_mkyong_core-java]
> 
> $ cd java-13

## 参考文献 ##

*  [OpenJDK 13项目][Java 13]
 *  [Oracle – Java 13的到来！][Oracle _ Java 13]
 *  [cl4cds][]
 *  [Java版本历史][Java 1]
 *  [Java 13开关表达式][Java 13 Switch Expressions]
 *  [具有应用程序类的Java 13-数据共享][Java 13 2]
 *  [Java 13文本块][Java 13 3]
 *  [Java安全性有哪些新功能？][Java 2]
 *  Java多行字符串，文本块示例

标签： [Java][Java 3] [Java 13][Java 13 4] [多行][Link 1] [新的JDK][JDK] [套接字][Link 2] [开关][Link 3] [文本blo][blo] [文本块][Link 4]

> 翻译自: [https://mkyong.com/java/what-is-new-in-java-13/][https_mkyong.com_java_what-is-new-in-java-13]

[Java]: http://imgconvert.csdnimg.cn/aHR0cHM6Ly9ta3lvbmcuY29tL3dwLWNvbnRlbnQvdXBsb2Fkcy8yMDE5LzA5L0phdmEtMTMtbG9nby5wbmc?x-oss-process=image/format,png
[Java 13]: https://openjdk.java.net/projects/jdk/13/
[Java 13 1]: https://jdk.java.net/java-se-ri/13
[openJDK]: https://jdk.java.net/archive/
[JEP 350_CDS]: https://openjdk.java.net/jeps/350
[JEP 351_ZGC]: https://openjdk.java.net/jeps/351
[JEP 353_API]: https://openjdk.java.net/jeps/353
[JEP 354]: https://openjdk.java.net/jeps/354
[JEP 355]: https://openjdk.java.net/jeps/355
[JEP 310 Application Class-Data Sharing]: https://openjdk.java.net/jeps/310
[cl4cds]: https://simonis.github.io/cl4cds/
[Java 13 2]: https://blog.codefx.org/java/application-class-data-sharing/
[JEP 333_Z]: https://openjdk.java.net/jeps/333
[JEP 325 Switch]: https://openjdk.java.net/jeps/325
[JEP 361]: https://openjdk.java.net/jeps/361
[Java 13 Switch Expressions]: https://www.mkyong.com/java/java-13-switch-expressions/
[Java 14]: https://openjdk.java.net/jeps/368
[https_github.com_mkyong_core-java]: https://github.com/mkyong/core-java
[Oracle _ Java 13]: https://blogs.oracle.com/java-platform-group/the-arrival-of-java-13
[Java 1]: https://en.wikipedia.org/wiki/Java_version_history
[Java 13 3]: https://blog.codefx.org/java/text-blocks/
[Java 2]: https://static.rainfocus.com/oracle/oow19/sess/1553278162777001mzA2/PF/DEV-3945-v10_1568859710817001R3n8.pdf
[Java 3]: https://mkyong.com/tag/java/
[Java 13 4]: https://mkyong.com/tag/java-13/
[Link 1]: https://mkyong.com/tag/multi-line/
[JDK]: https://mkyong.com/tag/new-jdk/
[Link 2]: https://mkyong.com/tag/socket/
[Link 3]: https://mkyong.com/tag/switch/
[blo]: https://mkyong.com/tag/text-blo/
[Link 4]: https://mkyong.com/tag/text-block/
[https_mkyong.com_java_what-is-new-in-java-13]: https://mkyong.com/java/what-is-new-in-java-13/