MethodHandles.Lookup (Java Platform SE 8 ) (original) (raw)

A lookup object is a factory for creating method handles, when the creation requires access checking. Method handles do not perform access checks when they are called, but rather when they are created. Therefore, method handle access restrictions must be enforced when a method handle is created. The caller class against which those restrictions are enforced is known as the lookup class.

A lookup class which needs to create method handles will callMethodHandles.lookup to create a factory for itself. When the Lookup factory object is created, the identity of the lookup class is determined, and securely stored in the Lookup object. The lookup class (or its delegates) may then use factory methods on the Lookup object to create method handles for access-checked members. This includes all methods, constructors, and fields which are allowed to the lookup class, even private ones.

Lookup Factory Methods

The factory methods on a Lookup object correspond to all major use cases for methods, constructors, and fields. Each method handle created by a factory method is the functional equivalent of a particular bytecode behavior. (Bytecode behaviors are described in section 5.4.3.5 of the Java Virtual Machine Specification.) Here is a summary of the correspondence between these factory methods and the behavior the resulting method handles:

Here, the type C is the class or interface being searched for a member, documented as a parameter named refc in the lookup methods. The method type MT is composed from the return type T and the sequence of argument types A*. The constructor also has a sequence of argument types A* and is deemed to return the newly-created object of type C. Both MT and the field type FT are documented as a parameter named type. The formal parameter this stands for the self-reference of type C; if it is present, it is always the leading argument to the method handle invocation. (In the case of some protected members, this may be restricted in type to the lookup class; see below.) The name arg stands for all the other method handle arguments. In the code examples for the Core Reflection API, the name thisOrNull stands for a null reference if the accessed method or field is static, and this otherwise. The names aMethod, aField, and aConstructor stand for reflective objects corresponding to the given members.

In cases where the given member is of variable arity (i.e., a method or constructor) the returned method handle will also be of variable arity. In all other cases, the returned method handle will be of fixed arity.

Discussion: The equivalence between looked-up method handles and underlying class members and bytecode behaviors can break down in a few ways:

• If C is not symbolically accessible from the lookup class's loader, the lookup can still succeed, even when there is no equivalent Java expression or bytecoded constant.
• Likewise, if T or MT is not symbolically accessible from the lookup class's loader, the lookup can still succeed. For example, lookups for MethodHandle.invokeExact andMethodHandle.invoke will always succeed, regardless of requested type.
• If there is a security manager installed, it can forbid the lookup on various grounds (see below). By contrast, the ldc instruction on a CONSTANT_MethodHandle constant is not subject to security manager checks.
• If the looked-up method has avery large arity, the method handle creation may fail, due to the method handle type having too many parameters.

Access checking

Access checks are applied in the factory methods of Lookup, when a method handle is created. This is a key difference from the Core Reflection API, sincejava.lang.reflect.Method.invoke performs access checking against every caller, on every call.

All access checks start from a Lookup object, which compares its recorded lookup class against all requests to create method handles. A single Lookup object can be used to create any number of access-checked method handles, all checked against a single lookup class.

A Lookup object can be shared with other trusted code, such as a metaobject protocol. A shared Lookup object delegates the capability to create method handles on private members of the lookup class. Even if privileged code uses the Lookup object, the access checking is confined to the privileges of the original lookup class.

A lookup can fail, because the containing class is not accessible to the lookup class, or because the desired class member is missing, or because the desired class member is not accessible to the lookup class, or because the lookup object is not trusted enough to access the member. In any of these cases, a ReflectiveOperationException will be thrown from the attempted lookup. The exact class will be one of the following:

• NoSuchMethodException — if a method is requested but does not exist
• NoSuchFieldException — if a field is requested but does not exist
• IllegalAccessException — if the member exists but an access check fails

In general, the conditions under which a method handle may be looked up for a method M are no more restrictive than the conditions under which the lookup class could have compiled, verified, and resolved a call to M. Where the JVM would raise exceptions like NoSuchMethodError, a method handle lookup will generally raise a corresponding checked exception, such as NoSuchMethodException. And the effect of invoking the method handle resulting from the lookup is exactly equivalent to executing the compiled, verified, and resolved call to M. The same point is true of fields and constructors.

Discussion: Access checks only apply to named and reflected methods, constructors, and fields. Other method handle creation methods, such asMethodHandle.asType, do not require any access checks, and are used independently of any Lookup object.

If the desired member is protected, the usual JVM rules apply, including the requirement that the lookup class must be either be in the same package as the desired member, or must inherit that member. (See the Java Virtual Machine Specification, sections 4.9.2, 5.4.3.5, and 6.4.) In addition, if the desired member is a non-static field or method in a different package, the resulting method handle may only be applied to objects of the lookup class or one of its subclasses. This requirement is enforced by narrowing the type of the leadingthis parameter from C (which will necessarily be a superclass of the lookup class) to the lookup class itself.

The JVM imposes a similar requirement on invokespecial instruction, that the receiver argument must match both the resolved method and the current class. Again, this requirement is enforced by narrowing the type of the leading parameter to the resulting method handle. (See the Java Virtual Machine Specification, section 4.10.1.9.)

The JVM represents constructors and static initializer blocks as internal methods with special names ("<init>" and "<clinit>"). The internal syntax of invocation instructions allows them to refer to such internal methods as if they were normal methods, but the JVM bytecode verifier rejects them. A lookup of such an internal method will produce a NoSuchMethodException.

In some cases, access between nested classes is obtained by the Java compiler by creating an wrapper method to access a private method of another class in the same top-level declaration. For example, a nested class C.D can access private members within other related classes such asC, C.D.E, or C.B, but the Java compiler may need to generate wrapper methods in those related classes. In such cases, a Lookup object onC.E would be unable to those private members. A workaround for this limitation is the Lookup.in method, which can transform a lookup on C.E into one on any of those other classes, without special elevation of privilege.

The accesses permitted to a given lookup object may be limited, according to its set of lookupModes, to a subset of members normally accessible to the lookup class. For example, the publicLookup method produces a lookup object which is only allowed to access public members in public classes. The caller sensitive method lookup produces a lookup object with full capabilities relative to its caller class, to emulate all supported bytecode behaviors. Also, the Lookup.in method may produce a lookup object with fewer access modes than the original lookup object.

Discussion of private access: We say that a lookup has private access if its lookup modes include the possibility of accessing private members. As documented in the relevant methods elsewhere, only lookups with private access possess the following capabilities:

• access private fields, methods, and constructors of the lookup class
• create method handles which invoke caller sensitive methods, such as Class.forName
• create method handles which emulate invokespecial instructions
• avoid package access checks for classes accessible to the lookup class
• create delegated lookup objects which have private access to other classes within the same package member

Each of these permissions is a consequence of the fact that a lookup object with private access can be securely traced back to an originating class, whose bytecode behaviors and Java language access permissions can be reliably determined and emulated by method handles.

Security manager interactions

Although bytecode instructions can only refer to classes in a related class loader, this API can search for methods in any class, as long as a reference to its Class object is available. Such cross-loader references are also possible with the Core Reflection API, and are impossible to bytecode instructions such as invokestatic or getfield. There is a security manager API to allow applications to check such cross-loader references. These checks apply to both the MethodHandles.Lookup API and the Core Reflection API (as found on Class).

If a security manager is present, member lookups are subject to additional checks. From one to three calls are made to the security manager. Any of these calls can refuse access by throwing aSecurityException. Define smgr as the security manager,lookc as the lookup class of the current lookup object,refc as the containing class in which the member is being sought, and defc as the class in which the member is actually defined. The value lookc is defined as not present if the current lookup object does not haveprivate access. The calls are made according to the following rules:

• Step 1: If lookc is not present, or if its class loader is not the same as or an ancestor of the class loader of refc, then smgr.checkPackageAccess(refcPkg) is called, where refcPkg is the package of refc.
• Step 2: If the retrieved member is not public andlookc is not present, thensmgr.checkPermission with RuntimePermission("accessDeclaredMembers") is called.
• Step 3: If the retrieved member is not public, and if lookc is not present, and if defc and refc are different, then smgr.checkPackageAccess(defcPkg) is called, where defcPkg is the package of defc.

Security checks are performed after other access checks have passed. Therefore, the above rules presuppose a member that is public, or else that is being accessed from a lookup class that has rights to access the member.

Caller sensitive methods

A small number of Java methods have a special property called caller sensitivity. A caller-sensitive method can behave differently depending on the identity of its immediate caller.

If a method handle for a caller-sensitive method is requested, the general rules for bytecode behaviors apply, but they take account of the lookup class in a special way. The resulting method handle behaves as if it were called from an instruction contained in the lookup class, so that the caller-sensitive method detects the lookup class. (By contrast, the invoker of the method handle is disregarded.) Thus, in the case of caller-sensitive methods, different lookup classes may give rise to differently behaving method handles.

In cases where the lookup object ispublicLookup(), or some other lookup object withoutprivate access, the lookup class is disregarded. In such cases, no caller-sensitive method handle can be created, access is forbidden, and the lookup fails with anIllegalAccessException.

Discussion: For example, the caller-sensitive methodClass.forName(x) can return varying classes or throw varying exceptions, depending on the class loader of the class that calls it. A public lookup of Class.forName will fail, because there is no reasonable way to determine its bytecode behavior.

If an application caches method handles for broad sharing, it should use publicLookup() to create them. If there is a lookup of Class.forName, it will fail, and the application must take appropriate action in that case. It may be that a later lookup, perhaps during the invocation of a bootstrap method, can incorporate the specific identity of the caller, making the method accessible.

The function MethodHandles.lookup is caller sensitive so that there can be a secure foundation for lookups. Nearly all other methods in the JSR 292 API rely on lookup objects to check access requests.