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Introduction

JAR file is a file format based on the popular ZIP file format and is used for aggregating many files into one. A JAR file is essentially a zip file that contains an optional META-INF directory. A JAR file can be created by the command-line 

jar tool, or by using the java.util.jar API in the Java platform. There is no restriction on the name of a JAR file, it can be any legal file name on a particular platform.

Modular JAR files

A modular JAR file is a JAR file that has a module descriptor, module-info.class, in the top-level directory (or root) directory. The module descriptor is the binary form of a module declaration. (Note the section on 

multi-release JAR files further refines the definition of modular JAR files.)

A modular JAR file deployed on the module path, as opposed to the class path, is an explicit module. Dependences and service providers are declared in the module descriptor. If the modular JAR file is deployed on the class path then it behaves as if a non-modular JAR file.

A non-modular JAR file deployed on the module path is an automatic module. If the JAR file has a main attribute Automatic-Module-Name (see Main Attributes) then the attribute's value is the module name, otherwise the module name is derived from the name of the JAR file as specified in ModuleFinder.of(Path...).

Multi-release JAR files

A multi-release JAR file allows for a single JAR file to support multiple major versions of Java platform releases. For example, a multi-release JAR file can depend on both the Java 8 and Java 9 major platform releases, where some class files depend on APIs in Java 8 and other class files depend on APIs in Java 9. This enables library and framework developers to decouple the use of APIs in a specific major version of a Java platform release from the requirement that all their users migrate to that major version. Library and framework developers can gradually migrate to and support new Java features while still supporting the old features.

A multi-release JAR file is identified by the main attribute:

Multi-Release: true

declared in the main section of the JAR Manifest.

Classes and resource files dependent on a major version, 9 or greater, of a Java platform release may be located under a versioned directory instead of under the top-level (or root) directory. The versioned directory is located under the the META-INF directory and is of the form:

META-INF/versions/N

where N is the string representation of the major version number of a Java platform release. Specifically N must conform to the specification:

N:	{1-9} {0-9}*

Any versioned directory whose value of N is less than 9 is ignored as is a string representation of N that does not conform to the above specification.

A class file under a versioned directory, of version N say, in a multi-release JAR must have a class file version less than or equal to the class file version associated with Nth major version of a Java platform release. If the class of the class file is public or protected then that class must preside over a class of the same fully qualified name and access modifier whose class file is present under the top-level directory. By logical extension this applies to a class of a class file, if present, under a versioned directory whose version is less than N.

If a multi-release JAR file is deployed on the class path or module path (as an automatic module or an explicit multi-release module) of major version N of a Java platform release runtime, then a class loader loading classes from that JAR file will first search for class files under the Nth versioned directory, then prior versioned directories in descending order (if present), down to a lower major version bound of 9, and finally under the top-level directory.

The public API exported by the classes in a multi-release JAR file must be exactly the same across versions, hence at a minimum why versioned public or protected classes for class files under a versioned directory must preside over classes for class files under the top-level directory. It is difficult and costly to perform extensive API verification checks as such tooling, such as the jar tool, is not required to perform extensive verification and a Java runtime is not required to perform any verification. A future release of this specification may relax the exact same API constraint to support careful evolution.

Resources under the META-INF directory cannot be versioned (such as for service configuration).

A multi-release JAR file can be signed.

Multi-release JAR files are not supported by the boot class loader of a Java runtime. If a multi-release JAR file is appended to the boot class path (with the -Xbootclasspath/a option) then the JAR is treated as if it is an ordinary JAR file.

Modular multi-release JAR files

A modular multi-release JAR file is a multi-release JAR file that has a module descriptor, module-info.class, in the top-level directory (as for a modular JAR file), or directly in a versioned directory.

A public or protected class in a non-exported package (that is not declared as exported in the module descriptor) need not preside over a class of the same fully qualified name and access modifier whose class file is present under the top-level directory.

A module descriptor is generally treated no differently to any other class or resource file. A module descriptor may be present under a versioned area but not present under the top-level directory. This ensures, for example, only Java 8 versioned classes can be present under the top-level directory while Java 9 versioned classes (including, or perhaps only, the module descriptor) can be present under the 9 versioned directory.

Any versioned module descriptor that presides over a lesser versioned module descriptor or that at the top-level, M say, must be identical to M, with two exceptions:

1. the presiding versioned descriptor can have different non-transitive requires clauses of java.* and jdk.* modules; and
2. the presiding versioned descriptor can have different uses clauses, even of service types defined outside of java.* and jdk.* modules.

Tooling, such as the jar tool, should perform such verification of versioned module descriptors but a Java runtime is not required to perform any verification.

The META-INF directory

The following files/directories in the META-INF directory are recognized and interpreted by the Java 2 Platform to configure applications, class loaders and services:

• MANIFEST.MF

The manifest file that is used to define package related data.

• INDEX.LIST

This file is generated by the new "-i" option of the jar tool, which contains location information for packages defined in an application. It is part of the JarIndex implementation and used by class loaders to speed up their class loading process.

• x.SF

The signature file for the JAR file. 'x' stands for the base file name.

• x.DSA

The signature block file associated with the signature file with the same base file name. This file stores the digital signature of the corresponding signature file.

• services/

This directory stores all the service provider configuration files for JAR files deployed on the class path or JAR files deployed as automatic modules on the module path. See the specification of service provider development for more details.

• versions/

This directory contains underneath it versioned class and resource files for a multi-release JAR file.

Name-Value pairs and Sections

Before we go to the details of the contents of the individual configuration files, some format convention needs to be defined. In most cases, information contained within the manifest file and signature files is represented as so-called "name: value" pairs inspired by the RFC822 standard. We also call these pairs headers or attributes.

Groups of name-value pairs are known as a "section". Sections are separated from other sections by empty lines.

Binary data of any form is represented as base64. Continuations are required for binary data which causes line length to exceed 72 bytes. Examples of binary data are digests and signatures.

Implementations shall support header values of up to 65535 bytes.

All the specifications in this document use the same grammar in which terminal symbols are shown in fixed width font and non-terminal symbols are shown in italic type face.

Specification:

section:	*header +newline
nonempty-section:	+header +newline
newline:	CR LF | LF | CR (not followed by LF)
header:	name : value
name:	alphanum *headerchar
value:	SPACE *otherchar newline *continuation
continuation:	SPACE *otherchar newline
alphanum:	{A-Z} | {a-z} | {0-9}
headerchar:	alphanum | - | _
otherchar:	any UTF-8 character except NUL, CR and LF

• Note: To prevent mangling of files sent via straight e-mail, no header will start with the four letters "From".

Non-terminal symbols defined in the above specification will be referenced in the following specifications.

JAR Manifest

Overview

A JAR file manifest consists of a main section followed by a list of sections for individual JAR file entries, each separated by a newline. Both the main section and individual sections follow the section syntax specified above. They each have their own specific restrictions and rules.

• The main section contains security and configuration information about the JAR file itself, as well as the application. It also defines main attributes that apply to every individual manifest entry. No attribute in this section can have its name equal to "Name". This section is terminated by an empty line.

• The individual sections define various attributes for packages or files contained in this JAR file. Not all files in the JAR file need to be listed in the manifest as entries, but all files which are to be signed must be listed. The manifest file itself must not be listed. Each section must start with an attribute with the name as "Name", and the value must be a relative path to the file, or an absolute URL referencing data outside the archive.

• If there are multiple individual sections for the same file entry, the attributes in these sections are merged. If a certain attribute have different values in different sections, the last one is recognized.

• Attributes which are not understood are ignored. Such attributes may include implementation specific information used by applications.

Manifest Specification:

manifest-file:	main-section newline *individual-section
main-section:	version-info newline *main-attribute
version-info:	Manifest-Version : version-number
version-number:	digit+{.digit+}*
main-attribute:	(any legitimate main attribute) newline
individual-section:	Name : value newline *perentry-attribute
perentry-attribute:	(any legitimate perentry attribute) newline
newline:	CR LF | LF | CR (not followed by LF)
digit:	{0-9}

In the above specification, attributes that can appear in the main section are referred to as main attributes, whereas attributes that can appear in individual sections are referred to as per-entry attributes. Certain attributes can appear both in the main section and the individual sections, in which case the per-entry attribute value overrides the main attribute value for the specified entry. The two types of attributes are defined as follows.

Main Attributes

Main attributes are the attributes that are present in the main section of the manifest. They fall into the following different groups:

• general main attributes
  • Manifest-Version: Defines the manifest file version. The value is a legitimate version number, as described in the above spec.
  • Created-By: Defines the version and the vendor of the java implementation on top of which this manifest file is generated. This attribute is generated by the jar tool.
  • Signature-Version: Defines the signature version of the jar file. The value should be a valid version-number string.
  • Class-Path: The value of this attribute specifies the relative URLs of the libraries that this application needs. URLs are separated by one or more spaces. The application class loader uses the value of this attribute to construct its internal search path. See Class-Path Attribute section for details.
  • Automatic-Module-Name: Defines the module name if this JAR file is deployed as an automatic module on the module path. For further details see the specification of automatic modules.
  • Multi-Release: This attribute defines whether this JAR file is a multi-release JAR file. If the value is "true" , case is ignored, then the JAR file will be processed by the Java runtime and tooling as a multi-release JAR file. Otherwise, if the value is anything other than "true" then this attribute is ignored.
• attribute defined for stand-alone applications: This attribute is used by stand-alone applications that are bundled into executable jar files which can be invoked by the java runtime directly by running "java -jar x.jar".
  • Main-Class: The value of this attribute is the class name of the main application class which the launcher will load at startup time. The value must not have the .class extension appended to the class name.
  • Launcher-Agent-Class: If this attribute is present then its value is the class name of a java agent that is started before the application main method is invoked. This attribute can be used for cases where a java agent is packaged in the same executable JAR file as the application. The agent class defines a public static method name agentmain in one of the two forms specified in the java.lang.instrumentpackage summary. Additional attributes (such as Can-Retransform-Classes) can be used to indicate capabilities needed by the agent.
• attributes defined for package versioning and sealing information: The value of these attributes apply to all the packages in the JAR file, but can be overridden by per-entry attributes.
  • Implementation-Title: The value is a string that defines the title of the extension implementation.
  • Implementation-Version: The value is a string that defines the version of the extension implementation.
  • Implementation-Vendor: The value is a string that defines the organization that maintains the extension implementation.
  • Specification-Title: The value is a string that defines the title of the extension specification.
  • Specification-Version: The value is a string that defines the version of the extension specification.
  • Specification-Vendor: The value is a string that defines the organization that maintains the extension specification.
  • Sealed: This attribute defines whether this JAR file is sealed or not. The value can be either "true" or "false", case is ignored. If it is set to "true", then all the packages in the JAR file are defaulted to be sealed, unless they are defined otherwise individually. See also the Package Sealing section.

Per-Entry Attributes

Per-entry attributes apply only to the individual JAR file entry to which the manifest entry is associated with. If the same attribute also appeared in the main section, then the value of the per-entry attribute overwrites the main attribute's value. For example, if JAR file a.jar has the following manifest content:

    Manifest-Version: 1.0
    Created-By: 1.8 (Oracle Inc.)
    Sealed: true
    Name: foo/bar/
    Sealed: false

It means that all the packages archived in a.jar are sealed, except that package foo.bar is not.

The per-entry attributes fall into the following groups:

• attributes defined for file contents:
  • Content-Type: This attribute can be used to specify the MIME type and subtype of data for a specific file entry in the JAR file. The value should be a string in the form of type/subtype. For example "image/bmp" is an image type with a subtype of bmp (representing bitmap). This would indicate the file entry as an image with the data stored as a bitmap. RFC 1521 and 1522 discuss and define the MIME types definition.
• attributes defined for package versioning and sealing information: These are the same set of attributes defined above as main attributes that defines the extension package versioning and sealing information. When used as per-entry attributes, these attributes overwrites the main attributes but only apply to the individual file specified by the manifest entry.
• attribute defined for beans objects:
  • Java-Bean: Defines whether the specific jar file entry is a Java Beans object or not. The value should be either "true" or "false", case is ignored.
• attributes defined for signing: These attributes are used for signing and verifying purposes. More details here.
  • x-Digest-y: The name of this attribute specifies the name of the digest algorithm used to compute the digest value for the corresponding jar file entry. The value of this attribute stores the actual digest value. The prefix 'x' specifies the algorithm name and the optional suffix 'y' indicates to which language the digest value should be verified against.
  • Magic: This is an optional attribute that can be used by applications to indicate how verifier should compute the digest value contained in the manifest entry. The value of this attribute is a set of comma separated context specific strings. Detailed description is here.

Signed JAR File

Overview

A JAR file can be signed by using the command line jarsigner tool or directly through the java.security API. Every file entry, including non-signature related files in the META-INF directory, will be signed if the JAR file is signed by the jarsigner tool. The signature related files are:

• META-INF/MANIFEST.MF
• META-INF/*.SF
• META-INF/*.DSA
• META-INF/*.RSA
• META-INF/SIG-*

Note that if such files are located in META-INF subdirectories, they are not considered signature-related. Case-insensitive versions of these filenames are reserved and will also not be signed.

Subsets of a JAR file can be signed by using the java.security API. A signed JAR file is exactly the same as the original JAR file, except that its manifest is updated and two additional files are added to the META-INFdirectory: a signature file and a signature block file. When jarsigner is not used, the signing program has to construct both the signature file and the signature block file.

For every file entry signed in the signed JAR file, an individual manifest entry is created for it as long as it does not already exist in the manifest. Each manifest entry lists one or more digest attributes and an optional Magic attribute.

Signature File

Each signer is represented by a signature file with extension .SF. The major part of the file is similar to the manifest file. It consists of a main section which includes information supplied by the signer but not specific to any particular jar file entry. In addition to the Signature-Version and Created-By attributes (see Main Attributes), the main section can also include the following security attributes:

• x-Digest-Manifest-Main-Attributes (where x is the standard name of a java.security.MessageDigest algorithm): The value of this attribute is the digest value of the main attributes of the manifest.
• x-Digest-Manifest (where x is the standard name of a java.security.MessageDigest algorithm): The value of this attribute is the digest value of the entire manifest.

The main section is followed by a list of individual entries whose names must also be present in the manifest file. Each individual entry must contain at least the digest of its corresponding entry in the manifest file.

Paths or URLs appearing in the manifest file but not in the signature file are not used in the calculation.

Signature Validation

A successful JAR file verification occurs if the signature(s) are valid, and none of the files that were in the JAR file when the signatures were generated have been changed since then. JAR file verification involves the following steps:

1. Verify the signature over the signature file when the manifest is first parsed. For efficiency, this verification can be remembered. Note that this verification only validates the signature directions themselves, not the actual archive files.

2. If an x-Digest-Manifest attribute exists in the signature file, verify the value against a digest calculated over the entire manifest. If more than one x-Digest-Manifest attribute exists in the signature file, verify that at least one of them matches the calculated digest value.

3. If an x-Digest-Manifest attribute does not exist in the signature file or none of the digest values calculated in the previous step match, then a less optimized verification is performed:

   1. If an x-Digest-Manifest-Main-Attributes entry exists in the signature file, verify the value against a digest calculated over the main attributes in the manifest file. If this calculation fails, then JAR file verification fails. This decision can be remembered for efficiency. If an x-Digest-Manifest-Main-Attributes entry does not exist in the signature file, its nonexistence does not affect JAR file verification and the manifest main attributes are not verified.

   2. Verify the digest value in each source file information section in the signature file against a digest value calculated against the corresponding entry in the manifest file. If any of the digest values don't match, then JAR file verification fails.

   One reason the digest value of the manifest file that is stored in the x-Digest-Manifest attribute may not equal the digest value of the current manifest file is that one or more files were added to the JAR file (using the jar tool) after the signature (and thus the signature file) was generated. When the jar tool is used to add files, the manifest file is changed (sections are added to it for the new files), but the signature file is not. A verification is still considered successful if none of the files that were in the JAR file when the signature was generated have been changed since then, which is the case if the digest values in the non-header sections of the signature file equal the digest values of the corresponding sections in the manifest file.

4. For each entry in the manifest, verify the digest value in the manifest file against a digest calculated over the actual data referenced in the "Name:" attribute, which specifies either a relative file path or URL. If any of the digest values don't match, then JAR file verification fails.

Example manifest file:

    Manifest-Version: 1.0
    Created-By: 1.8.0 (Oracle Inc.)

    Name: common/class1.class
    SHA-256-Digest: (base64 representation of SHA-256 digest)

    Name: common/class2.class
    SHA1-Digest: (base64 representation of SHA1 digest)
    SHA-256-Digest: (base64 representation of SHA-256 digest)

The corresponding signature file would be:

    Signature-Version: 1.0
    SHA-256-Digest-Manifest: (base64 representation of SHA-256 digest)
    SHA-256-Digest-Manifest-Main-Attributes: (base64 representation of SHA-256 digest)

    Name: common/class1.class
    SHA-256-Digest: (base64 representation of SHA-256 digest)

    Name: common/class2.class
    SHA-256-Digest: (base64 representation of SHA-256 digest)

The Magic Attribute

Another requirement to validate the signature on a given manifest entry is that the verifier understand the value or values of the Magic key-pair value in that entry's manifest entry.

The Magic attribute is optional but it is required that a parser understand the value of an entry's Magic key if it is verifying that entry's signature.

The value or values of the Magic attribute are a set of comma-separated context-specific strings. The spaces before and after the commas are ignored. Case is ignored. The exact meaning of the magic attributes is application specific. These values indicate how to compute the hash value contained in the manifest entry, and are therefore crucial to the proper verification of the signature. The keywords may be used for dynamic or embedded content, multiple hashes for multilingual documents, etc.

Here are two examples of the potential use of Magic attribute in the manifest file:

        Name: http://www.example-scripts.com/index#script1
        SHA-256-Digest: (base64 representation of SHA-256 hash)
        Magic: JavaScript, Dynamic

        Name: http://www.example-tourist.com/guide.html
        SHA-256-Digest: (base64 representation of SHA-256 hash)
        SHA-256-Digest-French: (base64 representation of SHA-256 hash)
        SHA-256-Digest-German: (base64 representation of SHA-256 hash)
        Magic: Multilingual

In the first example, these Magic values may indicate that the result of an http query is the script embedded in the document, as opposed to the document itself, and also that the script is generated dynamically. These two pieces of information indicate how to compute the hash value against which to compare the manifest's digest value, thus comparing a valid signature.

In the second example, the Magic value indicates that the document retrieved may have been content-negotiated for a specific language, and that the digest to verify against is dependent on which language the document retrieved is written in.

Digital Signatures

A digital signature is a signed version of the .SF signature file. These are binary files not intended to be interpreted by humans.

Digital signature files have the same filenames as the .SF files but different extensions. The extension varies depending on the type of digital signature.

• .RSA (PKCS7 signature, SHA-256 + RSA)
• .DSA (PKCS7 signature, DSA)

Digital signature files for signature algorithms not listed above must reside in the META-INF directory and have the prefix "SIG-". The corresonding signature file (.SF file) must also have the same prefix.

For those formats that do not support external signed data, the file shall consist of a signed copy of the .SF file. Thus some data may be duplicated and a verifier should compare the two files.

Formats that support external data either reference the .SF file, or perform calculations on it with implicit reference.

Each .SF file may have multiple digital signatures, but those signatures should be generated by the same legal entity.

File name extensions may be 1 to 3 alphanum characters. Unrecognized extensions are ignored.

Notes on Manifest and Signature Files

Following is a list of additional restrictions and rules that apply to manifest and signature files.

• Before parsing:
  • If the last character of the file is an EOF character (code 26), the EOF is treated as whitespace. Two newlines are appended (one for editors that don't put a newline at the end of the last line, and one so that the grammar doesn't have to special-case the last entry, which may not have a blank line after it).
• Attributes:
  • In all cases for all sections, attributes which are not understood are ignored.
  • Attribute names are case insensitive. Programs which generate manifest and signature files should use the cases shown in this specification however.
  • Attribute names cannot be repeated within a section.
• Versions:
  • Manifest-Version and Signature-Version must be first, and in exactly that case (so that they can be recognized easily as magic strings). Other than that, the order of attributes within a main section is not significant.
• Ordering:
  • The order of individual manifest entries is not significant.
  • The order of individual signature entries is not significant, except that the digests that get signed are in that order.
• Line length:
  • No line may be longer than 72 bytes (not characters), in its UTF8-encoded form. If a value would make the initial line longer than this, it should be continued on extra lines (each starting with a single SPACE).
• Errors:
  • If a file cannot be parsed according to this spec, a warning should be output, and none of the signatures should be trusted.
• Limitations:
  • Because header names cannot be continued, the maximum length of a header name is 70 bytes (there must be a colon and a SPACE after the name).
  • NUL, CR, and LF can't be embedded in header values, and NUL, CR, LF and ":" can't be embedded in header names.
  • Implementations should support 65535-byte (not character) header values, and 65535 headers per file. They might run out of memory, but there should not be hard-coded limits below these values.
• Signers:
  • It is technically possible that different entities may use different signing algorithms to share a single signature file. This violates the standard, and the extra signature may be ignored.
• Algorithms:
  • No digest algorithm or signature algorithm is mandated by this standard. However, at least one of SHA-256 and SHA1 digest algorithm must be supported.

JAR Index

Overview

Since 1.3, JarIndex is introduced to optimize the class searching process of class loaders for network applications, especially applets. Originally, an applet class loader uses a simple linear search algorithm to search each element on its internal search path, which is constructed from the "ARCHIVE" tag or the "Class-Path" main attribute. The class loader downloads and opens each element in its search path, until the class or resource is found. If the class loader tries to find a nonexistent resource, then all the jar files within the application or applet will have to be downloaded. For large network applications and applets this could result in slow startup, sluggish response and wasted network bandwidth. The JarIndex mechanism collects the contents of all the jar files defined in an applet and stores the information in an index file in the first jar file on the applet's class path. After the first jar file is downloaded, the applet class loader will use the collected content information for efficient downloading of jar files.

The existing jar tool is enhanced to be able to examine a list of jar files and generate directory information as to which classes and resources reside in which jar file. This directory information is stored in a simple text file named INDEX.LIST in the META-INF directory of the root jar file. When the classloader loads the root jar file, it reads the INDEX.LIST file and uses it to construct a hash table of mappings from file and package names to lists of jar file names. In order to find a class or a resource, the class loader queries the hashtable to find the proper jar file and then downloads it if necessary.

Once the class loader finds a INDEX.LIST file in a particular jar file, it always trusts the information listed in it. If a mapping is found for a particular class, but the class loader fails to find it by following the link, an unspecified Error or RuntimeException is thrown. When this occurs, the application developer should rerun the jar tool on the extension to get the right information into the index file.

To prevent adding too much space overhead to the application and to speed up the construction of the in-memory hash table, the INDEX.LIST file is kept as small as possible. For classes with non-null package names, mappings are recorded at the package level. Normally one package name is mapped to one jar file, but if a particular package spans more than one jar file, then the mapped value of this package will be a list of jar files. For resource files with non-empty directory prefixes, mappings are also recorded at the directory level. Only for classes with null package name, and resource files which reside in the root directory, will the mapping be recorded at the individual file level.

Index File Specification

The INDEX.LIST file contains one or more sections each separated by a single blank line. Each section defines the content of a particular jar file, with a header defining the jar file path name, followed by a list of package or file names, one per line. All the jar file paths are relative to the code base of the root jar file. These path names are resolved in the same way as the current extension mechanism does for bundled extensions.

The UTF-8 encoding is used to support non ASCII characters in file or package names in the index file.

Specification

index file:	version-info blankline section*
version-info:	JarIndex-Version: version-number
version-number:	digit+{.digit+}*
section:	body blankline
body:	header name*
header:	char+.jar newline
name:	char+ newline
char:	any valid Unicode character except NULL, CR andLF
blankline:	newline newline
newline:	CR LF | LF | CR (not followed by LF)
digit:	{0-9}

The INDEX.LIST file is generated by running jar -i. See the jar man page for more details.

Backward Compatibility

The new class loading scheme is totally backward compatible with applications developed on top of the current extension mechanism. When the class loader loads the first jar file and an INDEX.LIST file is found in the META-INF directory, it would construct the index hash table and use the new loading scheme for the extension. Otherwise, the class loader will simply use the original linear search algorithm.

Class-Path Attribute

The manifest for an application can specify one or more relative URLs referring to the JAR files and directories for other libraries that it needs. These relative URLs will be treated relative to the code base that the containing application was loaded from.

An application (or, more generally, JAR file) specifies the relative URLs of the libraries that it needs via the manifest attribute Class-Path. This attribute lists the URLs to search for implementations of other libraries if they cannot be found on the host Java Virtual Machine. These relative URLs may include JAR files and directories for any libraries or resources needed by the application. Relative URLs not ending with '/' are assumed to refer to JAR files. For example,

Class-Path: servlet.jar infobus.jar acme/beans.jar images/

At most one Class-Path header may be specified in a JAR file's manifest..

Currently, the URLs must be relative to the code base of the JAR file for security reasons. Thus, remote optional packages will originate from the same code base as the application.

Each relative URL is resolved against the code base that the containing application or library was loaded from. If the resulting URL is invalid or refers to a resource that cannot be found then it is ignored.

The resulting URLs are used to extend the class path for the application, applet, or servlet by inserting the URLs in the class path immediately following the URL of the containing JAR file. Any duplicate URLs are omitted. For example, given the following class path:

a.jar b.jar

If b.jar contained the following Class-Path manifest attribute:

Class-Path: x.jar a.jar

Then the resulting application class path would be the following:

a.jar b.jar x.jar

Of course, if x.jar had dependencies of its own then these would be added according to the same rules and so on for each subsequent URL. In the actual implementation, JAR file dependencies are processed lazily so that the JAR files are not actually opened until needed.

Package Sealing

JAR files and packages can be optionally sealed, so that an package can enforce consistency within a version.

A package sealed within a JAR specifies that all classes defined in that package must originate from the same JAR. Otherwise, a SecurityException is thrown.

A sealed JAR specifies that all packages defined by that JAR are sealed unless overridden specifically for a package.

A sealed package is specified via the manifest attribute, Sealed, whose value is true or false (case irrelevant). For example,

    Name: javax/servlet/internal/
    Sealed: true

specifies that the javax.servlet.internal package is sealed, and that all classes in that package must be loaded from the same JAR file.

If this attribute is missing, the package sealing attribute is that of the containing JAR file.

A sealed JAR is specified via the same manifest header, Sealed, with the value again of either true or false. For example,

    Sealed: true

specifies that all packages in this archive are sealed unless explicitly overridden for a particular package with the Sealed attribute in a manifest entry.

If this attribute is missing, the JAR file is assumed to not be sealed, for backwards compatibility. The system then defaults to examining package headers for sealing information.

Package sealing is also important for security, because it restricts access to package-protected members to only those classes defined in the package that originated from the same JAR file.

The unnamed package is not sealable, so classes that are to be sealed must be placed in their own packages.

官方中文介紹*******************************************************************************************

介紹

JAR檔案是基於流行的ZIP檔案格式的檔案格式，用於將許多檔案聚合為一個。JAR檔案本質上是一個包含可選META-INF目錄的zip檔案。JAR檔案可以通過命令列jar工具建立，也可以使用java.util.jarJava平臺中的API建立。對JAR檔案的名稱沒有限制，它可以是特定平臺上的任何合法檔名。

模組化JAR檔案

模組化JAR檔案是module-info.class在頂級目錄（或根目錄）目錄中具有模組描述符的JAR檔案。模組描述符是模組宣告的二進位制形式。（注意有關多版本JAR檔案的部分進一步細化了模組化JAR檔案的定義。）

部署在模組路徑上的模組化JAR檔案（與類路徑相對）是一個顯式模組。依賴項和服務提供程式在模組描述符中宣告。如果模組化JAR檔案部署在類路徑上，那麼它的行為就像非模組化JAR檔案一樣。

部署在模組路徑上的非模組化JAR檔案是自動模組。如果JAR檔案具有主屬性Automatic-Module-Name（請參閱主要屬性），則屬性的值是模組名稱，否則模組名稱是從中指定的JAR檔案的名稱派生的ModuleFinder.of(Path...)。

多版本JAR檔案

多版本JAR檔案允許單個JAR檔案支援多個主要版本的Java平臺版本。例如，多版本JAR檔案可以依賴於Java 8和Java 9主要平臺版本，其中一些類檔案依賴於Java 8中的API，而其他類檔案依賴於Java 9中的API。這使庫和框架開發人員成為可能將Java平臺版本的特定主要版本中的API的使用與其所有使用者遷移到該主要版本的要求分離。庫和框架開發人員可以逐步遷移並支援新的Java功能，同時仍支援舊功能。

主要屬性標識多版本JAR檔案：

Multi-Release: true

在JAR清單的主要部分宣佈。

依賴於Java平臺版本的主要版本（9或更高版本）的類和資原始檔可以位於版本化目錄下，而不是位於頂級（或根目錄）目錄下。版本化目錄位於META-INF目錄下，格式如下：

META-INF/versions/N

其中N是Java平臺發行版主要版本號的字串表示形式。具體N必須符合規範：

N：	{1-9} {0-9}*

任何版本化的目錄，其值N小於，將9被忽略，因為它的字串表示N不符合上述規範。

版本化目錄下的類檔案（N例如，在多版本JAR中）必須具有小於或等於與NJava平臺版本的主要版本相關聯的類檔案版本的類檔案版本。如果類檔案的類是公共的或受保護的，那麼該類必須主持一個相同的完全限定名稱和訪問修飾符的類，其類檔案存在於頂級目錄下。通過邏輯擴充套件，這適用於類檔案的類（如果存在），在版本小於的版本化目錄下N。

如果在Java平臺釋出執行時的主要版本的類路徑或模組路徑（作為自動模組或顯式多釋出模組）上部署多版本JAR檔案N，則從該JAR檔案載入類的類載入器將首先搜尋N版本化目錄下的類檔案，然後按降序排列先前版本的目錄（如果存在），下載到較低的主要版本9，最後搜尋頂級目錄。

多版本JAR檔案中的類匯出的公共API在不同版本中必須完全相同，因此至少為什麼版本化目錄下的類檔案的版本化公共類或受保護類必須主持類頂層下的類檔案的類 -級別目錄。執行廣泛的API驗證檢查既困難又昂貴，因為這樣的工具（例如jar工具）不需要執行大量驗證，並且不需要Java執行時來執行任何驗證。此規範的未來版本可能會放寬完全相同的API約束以支援細緻的演變。

META-INF目錄下的資源無法進行版本控制（例如用於服務配置）。

可以簽署多版本JAR檔案。

Java執行時的引導類載入器不支援多版本JAR檔案。如果將多版本JAR檔案附加到引導類路徑（使用該-Xbootclasspath/a選項），則將JAR視為普通JAR檔案。

模組化多版本JAR檔案

模組化多版本JAR檔案是一個多版本JAR檔案，它具有模組描述符module-info.class，位於頂級目錄（對於模組化 JAR檔案），或直接位於版本化目錄中。

非匯出包中的公共或受保護類（未宣告為在模組描述符中匯出）不需要主持一個相同的完全限定名稱和訪問修飾符的類，其類檔案存在於頂級目錄下。

模組描述符通常與任何其他類或資原始檔的處理方式沒有區別。模組描述符可以存在於版本化區域下但不存在於頂級目錄下。例如，這確保了只有Java 8版本化的類可以存在於頂級目錄下，而Java 9版本化的類（包括或者可能只是模組描述符）可以存在於9版本化目錄下。

任何版本化的模組描述符，主持較小的版本化模組描述符，或者在頂層，M例如，必須相同M，但有兩個例外：

1. 主持版本化描述符可以具有不同的transitive requires子句java.*和jdk.*模組; 和
2. 主持版本描述符可以具有不同的uses條款，甚至之外定義服務型別java.*和jdk.*模組。

工具（例如jar工具）應該對版本化模組描述符執行此類驗證，但不需要Java執行時來執行任何驗證。

META-INF目錄

Java 2平臺可識別和解釋META-INF目錄中的以下檔案/目錄，以配置應用程式，類載入器和服務：

• MANIFEST.MF

清單檔案，用於定義與包相關的資料。

• INDEX.LIST

此檔案由-i"jar工具的新選項生成，該選項包含應用程式中定義的包的位置資訊。它是JarIndex實現的一部分，並由類載入器用於加速其類載入過程。

• x.SF

JAR檔案的簽名檔案。'x'代表基本檔名。

• x.DSA

與簽名檔案關聯的簽名塊檔案具有相同的基本檔名。該檔案儲存相應簽名檔案的數字簽名。

• services/

此目錄儲存部署在類路徑上的JAR檔案的所有服務提供者配置檔案或部署為模組路徑上的自動模組的JAR檔案。有關更多詳細資訊，請參閱服務提供商開發規範。

• versions/

該目錄下面包含多版本 JAR檔案的版本化類和資原始檔。

名稱 - 值對和部分

在我們詳細介紹各個配置檔案的內容之前，需要定義一些格式約定。在大多數情況下，清單檔案和簽名檔案中包含的資訊表示為受RFC822標準啟發的所謂“名稱：值”對。我們還稱這些對標題或屬性。

名稱 - 值對的組稱為“部分”。截面用空行與其他部分分開。

任何形式的二進位制資料表示為base64。二進位制資料需要連續，這導致行長度超過72個位元組。二進位制資料的示例是摘要和簽名。

實現應支援最多65535位元組的標頭值。

本文件中的所有規範使用相同的語法，其中終端符號以固定寬度字型顯示，而非終端符號以斜體字型顯示。

規格：

部分：	*標題+換行符
非空段：	+標題+換行符
新隊：	CR LF | LF | CR（沒有跟著 LF）
標題：	名稱 : 值
名稱：	alphanum * headerchar
值：	SPACE * otherchar換行符*延續
延續：	SPACE * otherchar換行符
alphanum：	{ A-Z} | { a-z} | { 0-9}
headerchar：	alphanum | -|_
otherchar：	除 和之外的任何UTF-8字元NUL, CR LF

• 注意：為防止通過直接電子郵件傳送檔案，不會以四個字母“From”開頭。

上述規範中定義的非終端符號將在以下規範中引用。

JAR清單

概觀

JAR檔案清單包含一個主要部分，後跟各個JAR檔案條目的部分列表，每個部分由換行符分隔。主要部分和各個部分都遵循上面指定的部分語法。他們每個人都有自己的特定限制和規則。

• 主要部分包含有關JAR檔案本身以及應用程式的安全性和配置資訊。它還定義了適用於每個單獨清單條目的主要屬性。此部分中的任何屬性都不能使其名稱等於“ Name”。此部分以空行終止。

• 各個部分定義此JAR檔案中包含的包或檔案的各種屬性。並非JAR檔案中的所有檔案都需要作為條目列在清單中，但必須列出要簽名的所有檔案。不得列出清單檔案本身。每個部分必須以名稱為“ Name” 的屬性開頭，並且值必須是檔案的相對路徑，或者是引用存檔外部資料的絕對URL。

• 如果同一檔案條目有多個單獨的部分，則合併這些部分中的屬性。如果某個屬性在不同的部分中具有不同的值，則會識別最後一個屬性。

• 忽略不理解的屬性。這些屬性可以包括應用程式使用的實現特定資訊。

清單規格：

清單檔案：	主要部分換行*個別部分
主部：	version-info newline * main-attribute
版本資訊：	Manifest-Version : 版本號
版本號：	數字+ { .數字+} *
主要屬性：	（任何合法的主要屬性）換行符
個別部分：	Name : 值 換行* perentry-attribute
perentry屬性：	（任何合法的perentry屬性）換行符
新隊：	CR LF | LF | CR（沒有跟著 LF）
數字：	{0-9}

在上面的規範中，可以出現在主要部分中的屬性被稱為主要屬性，而可以出現在各個部分中的屬性被稱為每個條目屬性。某些屬性可以出現在主要部分和各個部分中，在這種情況下，每個條目屬性值將覆蓋指定條目的主要屬性值。兩種型別的屬性定義如下。

主要屬性

主要屬性是清單主要部分中存在的屬性。他們屬於以下不同的群體：

• 一般主要屬性
  • 清單 - 版本：定義清單檔案版本。該值是合法的