Java SE 21 & JDK 21

JAR File Specification

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 Platform to configure applications, class loaders and services:

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

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

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 in a PKCS #7 structure.

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.

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

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.

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:

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:

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:

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-INF directory: 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:

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 it might contain sections for newly added files after the file was signed. For example, suppose one or more files were added to the JAR file (using the jar tool) after the signature (and thus the signature file) was generated. If the JAR file is signed again by a different signer, then the manifest file is changed (sections are added to it for the new files by the jarsigner tool) and a new signature file is created, but the original signature file is unchanged. A verification on the original signature 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 algorithm of the signer's private key.

Digital signature files for signature algorithms not listed above must reside in the META-INF directory and have the prefix "SIG-". The corresponding 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.

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 (the "context JAR").

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.

A Class-Path entry is valid if the following conditions are true:

Invalid entries are ignored. Valid entries are resolved against the context JAR. If the resulting URL is invalid or refers to a resource that cannot be found, then it is ignored. Duplicate URLs are ignored.

The resulting URLs are inserted into the class path, immediately following the URL of the context JAR. For example, given the following class path:

a.jar b.jar

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

Class-Path: lib/x.jar a.jar

Then the effective search path of such a URLClassLoader instance would be:

a.jar b.jar lib/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.

API Details

Package java.util.jar

See Also

Package java.security
Package java.util.zip