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Java 平台标准版 Oracle JDK 9 迁移指南概述： Oracle JDK 9 迁移指南旨在帮助开发者识别潜在的问题，并提供了解决方案，以便将现有的 Java 应用程序迁移到 JDK 9。本指南涵盖了 Java SE 版本的变化、移除或更改的 API、垃圾回收、安全更新、部署等多个方面，旨在帮助开发者顺利地迁移到 JDK 9。一、准备迁移下载 JDK 9 并安装，然后尝试运行应用程序，以了解可能出现的问题。如果应用程序无法运行，需要检查错误日志，了解具体的错误信息。二、新版本字符串方案 JDK 9 引入了新的版本字符串方案，以便更好地标识 Java 版本。本节介绍了新的版本字符串方案的格式和使用方法。三、运行时访问警告 JDK 9 引入了运行时访问警告机制，以便警告开发者可能的不兼容问题。本节介绍了运行时访问警告的机制和使用方法。四、JDK/JRE 映像的变化 JDK 9 的安装程序现在使用新的映像格式，以便提高安装速度和可靠性。本节介绍了 JDK/JRE 映像的变化和使用方法。五、移除或更改的 API JDK 9 移除了或更改了一些 API，以提高 Java 平台的安全性和可靠性。本节介绍了移除或更改的 API 列表和使用方法。六、共享模块的变化 JDK 9 引入了模块化机制，以便提高 Java 平台的可靠性和灵活性。本节介绍了共享模块的变化和使用方法。七、部署 JDK 9 引入了新的部署机制，以便提高应用程序的可靠性和灵活性。本节介绍了部署的变化和使用方法。八、安全更新 JDK 9 引入了新的安全更新机制，以便提高 Java 平台的安全性。本节介绍了安全更新的机制和使用方法。九、垃圾回收的变化 JDK 9 引入了新的垃圾回收机制，以便提高应用程序的性能和可靠性。本节介绍了垃圾回收的变化和使用方法。十、移除的工具和组件 JDK 9 移除了的一些工具和组件，以便提高 Java 平台的简洁性和可靠性。本节介绍了移除的工具和组件列表和使用方法。十一、macOS 特定的功能移除 JDK 9 移除了 macOS 特定的功能，以便提高 Java 平台的可移植性和可靠性。本节介绍了移除的 macOS 特定的功能列表和使用方法。十二、展望未来 JDK 9 的发布标志着 Java 平台的新时代。本节介绍了 JDK 9 的新功能和使用方法，并为开发者提供了展望未来的一些建议。

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Java Platform, Standard Edition

Oracle JDK 9 Migration Guide

Release 9

E82965-05

October 2017

Migrating to JDK 9

The purpose of this guide is to help you identify potential issues and give you

suggestions on how to proceed as you migrate your existing Java application to JDK

Every new Java SE release introduces some binary, source, and behavioral

incompatibilities with previous releases. The modularization of the Java SE Platform

brings many benefits, but also many changes. Code that uses only official Java SE

Platform APIs and supported JDK-specific APIs should continue to work without

change. Code that uses JDK-internal APIs should continue to run but should be

migrated to use external APIs.

To migrate your application, start by following the steps listed in Prepare for Migration.

Then, look at the list of changes that you may encounter as you run your application.

• New Version-String Scheme

• Understanding Runtime Access Warnings

• Changes to the Installed JDK/JRE Image

• Removed or Changed APIs

• Modules Shared with Java EE Not Resolved by Default

• Deployment

• Security Updates

• Changes to Garbage Collection

• Removed Tools and Components

• Removed macOS-Specific Features

Finally, when your application is running successfully on JDK 9, review Looking

Forward, which will help you avoid problems with future releases.

This guide focuses on changes required to make your code run on JDK 9. For a

comprehensive list of all of the new features of JDK 9, see Java Platform, Standard

Edition What's New in JDK 9 For detailed information about the changes mentioned in

this guide, refer to JDK 9 Release Notes.

Check the websites for your third-party libraries and your tool vendors for a version of

each library or tool that’s designed to work on JDK 9. If one exists, then download and

install the new version.

If you use Maven or Gradle to build your application, then make sure to upgrade to a

more recent version that supports JDK 9.

If you use an IDE to develop your applications, then it can help to migrate existing

code. The NetBeans, Eclipse, and IntelliJ IDEs all have versions available that include

JDK 9 support.

You can see the status of the testing of many Free Open Source Software (FOSS)

projects with OpenJDK builds at Quality Outreach on the OpenJDK wiki.

Compile Your Application if Needed

Compiling your code with the JDK 9 compiler will ease migration to future releases

since the code may depend on APIs and features which have been identified as

problematic. However, it is not strictly necessary.

If you need to compile your code with the JDK 9 compiler then take note of the

following:

• If you use the underscore character ("_") as a one-character identifier in source

code, then your code won’t compile in JDK 9. Its use generates a warning in JDK

8, and an error in JDK 9.

As an example:

static Object _ = new Object();

This code generates the following error message from the compiler:

MyClass.java:2: error: as of release 9, '_' is a keyword, and may not be used as

a legal identifier.

• If you use the

-source

and

-target

options with

javac

, then check the values that

you use. In JDK 9,

javac

uses a "one plus three back" policy of supporting

-source

and

-target

options.

The supported

-source/-target

values are 9 (the default), 8, 7, and 6 (6 is

deprecated, and a warning is displayed when this value is used).

In JDK 8,

-source

and

-target

values of 1.5/5 and earlier were deprecated and

caused a warning to be generated. In JDK 9, those values cause an error.

>javac -source 5 -target 5 Sample.java

warning: [options] bootstrap class path not set in conjunction with -source 1.5

error: Source option 1.5 is no longer supported. Use 1.6 or later.

error: Target option 1.5 is no longer supported. Use 1.6 or later.

If possible, use the new

--release

flag instead of the

-source

and

-target

options.

The

--release N

flag is conceptually a macro for:

-source N -target N -bootclasspath $PATH_TO_rt.jar_FOR_RELEASE_N

The valid arguments for the

--release

flag follow the same policy as for

-source

and

-target

, one plus three back.

javac

can recognize and process class files of all previous JDKs, going all the way

back to JDK 1.0.2 class files.

See JEP 182: Policy for Retiring javac -source and -target Options.

• Critical internal JDK APIs such as sun.misc.Unsafe are still accessible in JDK

9, but most of the JDK’s internal APIs are not accessible at compile time. You may

get compilation errors that indicate that your application or its libraries are

dependent on internal APIs.

To identify the dependencies, run the Java Dependency Analysis tool. See Run

jdeps on Your Code. If possible, update your code to use the supported

replacement APIs.

You may use the

--add-exports

option as a temporary workaround to compile

source code with references to JDK internal classes.

• You may see more deprecation warnings than previously. If you see deprecation

with removal warnings, then you should address those to avoid future problems.

• A number of small changes have been made to

javac

to align it with the Java SE 9

Language Specification.

• You may encounter some source compatibility issues when recompiling.

The JDK 9 Release Notes contains details of changes to the

javac

compiler and

source compatibility issues in JDK 9.

Run jdeps on Your Code

Run the

jdeps

tool on your application to see what packages and classes your

applications and libraries depend on. If you use internal APIs, then

jdeps

may suggest

replacements to help you to update your code.

To look for dependencies on internal JDK APIs, run

jdeps

with the

-jdkinternals

option. For example, if you run

jdeps

on a class that calls

sun.misc.BASE64Encoder

you’ll see:

>jdeps -jdkinternals Sample.class

Sample.class -> JDK removed internal API

Sample -> sun.misc.BASE64Encoder JDK internal API (JDK removed internal API)

Warning: JDK internal APIs are unsupported and private to JDK implementation that are

subject to be removed or changed incompatibly and could break your application.

Please modify your code to eliminate dependency on any JDK internal APIs.

For the most recent update on JDK internal API replacements, please check:

https://wiki.openjdk.java.net/display/JDK8/Java+Dependency+Analysis+Tool

JDK Internal API Suggested Replacement

---------------- ---------------------

sun.misc.BASE64Encoder Use java.util.Base64 @since 1.8

If you use Maven, there’s a

jdeps

plugin available.

For

jdeps

syntax, see

jdeps

in the Java Platform, Standard Edition Tools Reference.

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