此电脑-》属性-》高级系统设置-》环境变量

之前的环境变量设置为：

JAVA_HOME：jdk包的本地路径
Path：;%JAVA_HOME%\bin;%JAVA_HOME%\jre\bin
CLASSPATH：.;%JAVA_HOME%\lib\dt.jar;%JAVA_HOME%\lib\tools.jar

注意在 Java9 找不到 dt.jar 和 tools.jar 了，因而有了一些变化：

JAVA_HOME：jdk包的本地路径
JRE_HOME：jre包的本地路径
Path：%JAVA_HOME%\bin;%JRE_HOME%\bin
CLASSPATH：.;%JAVA_HOME%\lib;%JRE_HOME%\lib

测试：
在cmd中测试这两个命令查看是否正确安装和正确设置：

java -version
javac

然后启动 eclipse 遇到错误：

eclipse.buildId=4.7.0.I20170612-0950
java.version=9
java.vendor=Oracle Corporation
BootLoader constants: OS=win32, ARCH=x86_64, WS=win32, NL=zh_CN
Framework arguments: -product org.eclipse.epp.package.jee.product
Command-line arguments: -os win32 -ws win32 -arch x86_64 -product org.eclipse.epp.package.jee.product

!ENTRY org.eclipse.osgi 4 0 2017-09-26 13:16:34.647
!MESSAGE Application error
!STACK 1
org.eclipse.e4.core.di.InjectionException: java.lang.NoClassDefFoundError: javax/annotation/PostConstruct
at org.eclipse.e4.core.internal.di.InjectorImpl.internalMake(InjectorImpl.java:410)
at org.eclipse.e4.core.internal.di.InjectorImpl.make(InjectorImpl.java:318)
at org.eclipse.e4.core.contexts.ContextInjectionFactory.make(ContextInjectionFactory.java:162)
at org.eclipse.e4.ui.internal.workbench.swt.E4Application.createDefaultHeadlessContext(E4Application.java:491)
at org.eclipse.e4.ui.internal.workbench.swt.E4Application.createDefaultContext(E4Application.java:505)
at org.eclipse.e4.ui.internal.workbench.swt.E4Application.createE4Workbench(E4Application.java:204)
at org.eclipse.ui.internal.Workbench.lambda$3(Workbench.java:614)
at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:336)
at org.eclipse.ui.internal.Workbench.createAndRunWorkbench(Workbench.java:594)
at org.eclipse.ui.PlatformUI.createAndRunWorkbench(PlatformUI.java:148)
at org.eclipse.ui.internal.ide.application.IDEApplication.start(IDEApplication.java:151)
at org.eclipse.equinox.internal.app.EclipseAppHandle.run(EclipseAppHandle.java:196)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.runApplication(EclipseAppLauncher.java:134)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.start(EclipseAppLauncher.java:104)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:388)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:243)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.base/java.lang.reflect.Method.invoke(Unknown Source)
at org.eclipse.equinox.launcher.Main.invokeFramework(Main.java:653)
at org.eclipse.equinox.launcher.Main.basicRun(Main.java:590)
at org.eclipse.equinox.launcher.Main.run(Main.java:1499)
at org.eclipse.equinox.launcher.Main.main(Main.java:1472)
Caused by: java.lang.NoClassDefFoundError: javax/annotation/PostConstruct
at org.eclipse.e4.core.internal.di.InjectorImpl.inject(InjectorImpl.java:124)
at org.eclipse.e4.core.internal.di.InjectorImpl.internalMake(InjectorImpl.java:399)
... 23 more
Caused by: java.lang.ClassNotFoundException: javax.annotation.PostConstruct cannot be found by org.eclipse.e4.core.di_1.6.100.v20170421-1418
at org.eclipse.osgi.internal.loader.BundleLoader.findClassInternal(BundleLoader.java:433)
at org.eclipse.osgi.internal.loader.BundleLoader.findClass(BundleLoader.java:395)
at org.eclipse.osgi.internal.loader.BundleLoader.findClass(BundleLoader.java:387)
at org.eclipse.osgi.internal.loader.ModuleClassLoader.loadClass(ModuleClassLoader.java:150)
at java.base/java.lang.ClassLoader.loadClass(Unknown Source)
... 25 more

解决方案：
在 eclipse.ini 中添加

--add-modules=java.se.ee

参考：https://stackoverflow.com/questions/34947994/neon-how-to-run-on-jdk9/34948255#34948255