JDK 8 tools.jar is the JAR file for JDK 8 tools. It contains Java classes to support different JDK tool commands:

sun.applet.Main - Supports the "appletviewer" command.
sun.rmi.rmic.Main - Supports the "rmic" command.
sun.security.tools.jarsigner.Main - Supports the "jarsigner" command.
sun.tools.jar.Main - Supports the "jar" command.
sun.tools.javac.Main - Supports the "javac" command. 
sun.tools.jcmd.JCmd - Supports the "jcmd" command.
sun.tools.jinfo.JInfo - Supports the "jinfo" command.
sun.tools.jmap.JMap - Supports the "jmap" command.
sun.tools.jps.Jps - Supports the "jps" command.
sun.tools.jstack.JStack - Supports the "jstack" command.
sun.tools.jstat.Jstat - Supports the "jstat" command.
sun.tools.jstatd.Jstatd - Supports the "jstatd" command.
sun.tools.native2ascii.Main - Supports the "native2ascii" command.
sun.tools.serialver.SerialVer - Supports the "serialver" command.
... 

JAR File Information:

Directory of C:\fyicenter\jdk-1.8.0_191\lib
      18,256,125 tools.jar

Package Content:

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com/sun/tools/javac/comp/ConstFold.class
com/sun/tools/javac/comp/DeferredAttr.class
com/sun/tools/javac/comp/Enter.class
com/sun/tools/javac/comp/Env.class
com/sun/tools/javac/comp/Flow.class
com/sun/tools/javac/comp/Infer.class
com/sun/tools/javac/comp/LambdaToMethod.class
com/sun/tools/javac/comp/Lower.class
com/sun/tools/javac/comp/MemberEnter.class
com/sun/tools/javac/comp/Resolve.class
com/sun/tools/javac/comp/Todo.class
com/sun/tools/javac/comp/TransTypes.class
com/sun/tools/javac/comp/TypeEnvs.class
com/sun/tools/javac/file/BaseFileObject.class
com/sun/tools/javac/file/CacheFSInfo.class
com/sun/tools/javac/file/FSInfo.class
com/sun/tools/javac/file/JavacFileManager.class
com/sun/tools/javac/file/Locations.class
com/sun/tools/javac/file/RegularFileObject.class
com/sun/tools/javac/file/RelativePath.class
com/sun/tools/javac/file/SymbolArchive.class
com/sun/tools/javac/file/ZipArchive.class
com/sun/tools/javac/file/ZipFileIndex.class
com/sun/tools/javac/file/ZipFileIndexArchive.class
com/sun/tools/javac/file/ZipFileIndexCache.class
com/sun/tools/javac/jvm/ByteCodes.class
com/sun/tools/javac/jvm/ClassFile.class
com/sun/tools/javac/jvm/ClassReader.class
com/sun/tools/javac/jvm/ClassWriter.class
com/sun/tools/javac/jvm/Code.class
com/sun/tools/javac/jvm/CRTable.class
com/sun/tools/javac/jvm/CRTFlags.class
com/sun/tools/javac/jvm/Gen.class
com/sun/tools/javac/jvm/Items.class
com/sun/tools/javac/jvm/JNIWriter.class
com/sun/tools/javac/jvm/Pool.class
com/sun/tools/javac/jvm/Profile.class
com/sun/tools/javac/jvm/Target.class
com/sun/tools/javac/jvm/UninitializedType.class
com/sun/tools/javac/main/CommandLine.class
com/sun/tools/javac/main/JavaCompiler.class
com/sun/tools/javac/main/Main.class
com/sun/tools/javac/main/Option.class
com/sun/tools/javac/main/OptionHelper.class
com/sun/tools/javac/Main.class
com/sun/tools/javac/model/AnnotationProxyMaker.class
com/sun/tools/javac/model/FilteredMemberList.class
com/sun/tools/javac/model/JavacElements.class
com/sun/tools/javac/model/JavacSourcePosition.class
com/sun/tools/javac/model/JavacTypes.class
com/sun/tools/javac/nio/JavacPathFileManager.class
com/sun/tools/javac/nio/PathFileManager.class
com/sun/tools/javac/nio/PathFileObject.class
com/sun/tools/javac/parser/DocCommentParser.class
com/sun/tools/javac/parser/JavacParser.class
com/sun/tools/javac/parser/JavadocTokenizer.class
com/sun/tools/javac/parser/JavaTokenizer.class
com/sun/tools/javac/parser/LazyDocCommentTable.class
com/sun/tools/javac/parser/Lexer.class
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⇒ JRE 8 charsets.jar - Character Set Encoding

⇐ JDK 8 sa-jdi.jar - HSDB (Hotspot Debugger)

⇑ Download and Use JDK 8

⇑⇑ FAQ for JDK (Java Development Kit)