What is the lib\modules file in JDK 9 installation directory?

lib\modules file in JDK 9 installation directory is the runtime JImage file of all Java language modules and all JDK tool modules.

You can list all modules and classes in lib\modules using the "jimage list" command:

C:>dir \fyicenter\jdk-9.0.4\bin\jimage list \fyicenter\jdk-9.0.4\lib\modules

jimage: \fyicenter\jdk-9.0.4\lib\modules

Module: java.activation
    META-INF/mailcap.default
    META-INF/mimetypes.default
    com/sun/activation/registries/LineTokenizer.class
    com/sun/activation/registries/LogSupport.class
    com/sun/activation/registries/MailcapFile.class
    com/sun/activation/registries/MailcapParseException.class
    com/sun/activation/registries/MailcapTokenizer.class
    com/sun/activation/registries/MimeTypeEntry.class
    com/sun/activation/registries/MimeTypeFile.class
    javax/activation/ActivationDataFlavor.class
    javax/activation/CommandInfo$Beans$1.class
    javax/activation/CommandInfo$Beans.class
    javax/activation/CommandInfo.class
    javax/activation/CommandMap.class
    javax/activation/CommandObject.class
    javax/activation/DataContentHandler.class
    javax/activation/DataContentHandlerFactory.class
    javax/activation/DataHandler$1.class
    javax/activation/DataHandler.class
    javax/activation/DataHandlerDataSource.class
    javax/activation/DataSource.class
    javax/activation/DataSourceDataContentHandler.class
    javax/activation/FileDataSource.class
    javax/activation/FileTypeMap.class
    javax/activation/MailcapCommandMap$1.class
    javax/activation/MailcapCommandMap.class
    javax/activation/MimeType.class
    javax/activation/MimeTypeParameterList.class
    javax/activation/MimeTypeParseException.class
    javax/activation/MimetypesFileTypeMap$1.class
    javax/activation/MimetypesFileTypeMap.class
    javax/activation/ObjectDataContentHandler.class
    javax/activation/SecuritySupport$1.class
    javax/activation/SecuritySupport$2.class
    javax/activation/SecuritySupport$3.class
    javax/activation/SecuritySupport$4.class
    javax/activation/SecuritySupport$5.class
    javax/activation/SecuritySupport.class
    javax/activation/URLDataSource.class
    javax/activation/UnsupportedDataTypeException.class
    module-info.class

Module: java.base
    META-INF/services/java.nio.file.spi.FileSystemProvider
    com/sun/crypto/provider/AESCipher$AES128_CBC_NoPadding.class
    com/sun/crypto/provider/AESCipher$AES128_CFB_NoPadding.class
    ...

⇒ JDK 9 lib/src.zip - Source Code

⇐ JDK 9 jmods/*.jmod - List of JMod Files

⇑ Download and Use JDK 9

⇑⇑ FAQ for JDK (Java Development Kit)