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# Garbage Collector Interfaces

The interfaces and classes in this chapter are used by the distributed garbage collector (DGC)for RMI.

### Topics:

- The Interface DGC
- The Lease Class
- The ObjID Class
- The UID Class
- The VMID Class

# The Interface DGC

The DGC abstraction is used for the server side of the distributed garbage collection algorithm. This interface contains the two methods: `dirty` and `clean`. A `dirty` call is made when a remote reference is unmarshaled in a client (the client is indicated by its `VMID`). A corresponding clean call is made when no more references to the remote reference exist in the client. A failed dirty call must schedule a strong clean call so that the call's sequence number can be retained in order to detect future calls received out of order by the distributed garbage collector.

A reference to a remote object is leased for a period of time by the client holding the reference. The lease period starts when the dirty call is received. It is the client's responsibility to renew the leases, by making additional `dirty` calls, on the remote references it holds before such leases expire. If the client does not renew the lease before it expires, the distributed garbage collector assumes that the remote object is no longer referenced by that client.

    package java.rmi.dgc;
    import java.rmi.server.ObjID;

    public interface DGC extends java.rmi.Remote {

    	Lease dirty(ObjID[] ids, long sequenceNum, Lease lease)
    		throws java.rmi.RemoteException;

    	void clean(ObjID[] ids, long seqNum, VMID vmid, boolean strong)
    		throws java.rmi.RemoteException;
    }

The method `dirty` requests leases for the remote object references associated with the object identifiers contained in the array argument ids. The lease contains a client's unique virtual machine identifier (VMID) and a requested lease period. For each remote object exported in the local virtual machine, the garbage collector maintains a reference list - a list of clients that hold references to it. If the lease is granted, the garbage collector adds the client's VMID to the reference list for each remote object indicated in ids. The sequenceNum parameter is a sequence number that is used to detect and discard late calls to the garbage collector. The sequence number should always increase for each subsequent call to the garbage collector.

Some clients are unable to generate a unique VMID. This is because a VMID is a universally unique identifier only if it contains a true host address, an address which some clients are unable to obtain due to security restrictions. In this case, a client can use a VMID of `null`, and the distributed garbage collector will assign a VMID for the client.

The `dirty` call returns a `Lease` object that contains the VMID used and the lease period granted for the remote references. (A server can decide to grant a smaller lease period than the client requests.) A client must use the VMID the garbage collector uses in order to make corresponding `clean` calls when the client drops remote object references.

A client virtual machine need only make one initial `dirty` call for each remote reference referenced in the virtual machine (even if it has multiple references to the same remote object). The client must also make a `dirty` call to renew leases on remote references before such leases expire. When the client no longer has any references to a specific remote object, it must schedule a `clean` call for the object ID associated with the reference.

The `clean` call removes the vmid from the reference list of each remote object indicated in ids. The sequence number is used to detect late clean calls. If the argument strong is true, then the clean call is a result of a failed `dirty` call, and the sequence number for the client vmid therefore needs to be remembered.

# The Lease Class

A lease contains a unique virtual machine identifier and a lease duration. A `Lease` object is used to request and grant leases to remote object references.

    package java.rmi.dgc;

    public final class Lease implements java.io.Serializable {

    	public Lease(VMID id, long duration);

    	public VMID getVMID();

    	public long getValue();
    }

The `Lease` constructor creates a lease with a specific VMID and lease duration. The VMID may be `null`.

The `getVMID` method returns the client VMID associated with the lease.

The `getValue` method returns the lease duration.

# The ObjID Class

The class `ObjID` is used to identify remote objects uniquely in a virtual machine over time. Each identifier contains an object number and an address space identifier that is unique with respect to a specific host. An object identifier is assigned to a remote object when it is exported.

An `ObjID` consists of an object number (a `long`) and a unique identifier for the address space (a `UID`).

    package java.rmi.server;

    public final class ObjID implements java.io.Serializable {

    	public ObjID ();

    	public ObjID (int num);

    	public void write(ObjectOutput out) throws java.io.IOException;

    	public static ObjID read(ObjectInput in)
    		throws java.io.IOException;

    	public int hashCode()

    	public boolean equals(Object obj)

    	public String toString()
    }

The first form of the `ObjID` constructor generates a unique object identifier. The second constructor generates well-known object identifiers (such as those used by the registry and the distributed garbage collector) and takes as an argument a well-known object number. A well-known object ID generated via this second constructor will not clash with any object IDs generated via the default constructor; to enforce this, the object number of the `ObjID` is set to the "well-known" number supplied in the constructor and all `UID` fields are set to zero.

The method `write` marshals the object ID's representation to an output stream.

The method `read` constructs an object ID whose contents is read from the specified input stream.

The method `hashCode` returns the object number as the hashcode

The `equals` method returns true if obj is an `ObjID` with the same contents.

The `toString` method returns a string containing the object ID representation. The address space identifier is included in the string representation only if the object ID is from a non-local address space.

# The UID Class

The class `UID` is an abstraction for creating identifiers that are unique with respect to the host on which it is generated. A `UID` is contained in an `ObjID` as an address space identifier. A UID consists of a number that is unique on the host (an `int`), a time (a `long`), and a count (a `short`).

    package java.rmi.server;

    public final class UID implements java.io.Serializable {

        public UID();

        public UID(short num);

        public int hashCode();

        public boolean equals(Object obj);

        public String toString();

        public void write(DataOutput out) throws java.io.IOException;

        public static UID read(DataInput in) throws java.io.IOException;
    }

The first form of the constructor creates a pure identifier that is unique with respect to the host on which it is generated. This `UID` is unique under the following conditions: a) the machine takes more than one second to reboot, and b) the machine's clock is never set backward. In order to construct a `UID` that is globally unique, simply pair a `UID` with an `InetAddress`.

The second form of the constructor creates a well-known `UID`. There are 216 -1 such possible well-known IDs. An ID generated via this constructor will not clash with any ID generated via the default `UID` constructor which generates a genuinely unique identifier with respect to this host.

The methods `hashCode`, `equals`, and `toString` are defined for UIDs. Two UIDs are considered equal if they have the same contents.

The method `write` writes the UID to the output stream.

The method `read` constructs a UID whose contents is read from the specified input stream.

# The VMID Class

The class `VMID` provides a universally unique identifier among all Java virtual machines. A `VMID` contains a `UID` and a host address. A `VMID` can be used to identify client virtual machines.

    package java.rmi.dgc;

    public final class VMID implements java.io.Serializable {

    	public VMID();

    	public static boolean isUnique();

    	public int hashCode();

    	public boolean equals(Object obj);

    	public String toString();
    }

The `VMID` default constructor creates a globally unique identifier among all Java virtual machines under the following conditions:

- the conditions for uniqueness for objects of the class `java.rmi.server.UID` are satisfied, and
- an address can be obtained for the host that is unique and constant for the lifetime of the `UID` object.

A `VMID` contains the host address of the machine on which it was created. Due to security restrictions, obtaining the true host address is not always possible (for example, the loopback host may be used under security-restricted conditions). The method `isUnique` can be called to determine if `VMIDs` generated in this virtual machine are, in fact, unique among all virtual machines. The method `isUnique` returns true if a valid host name can be determined (other than loopback host); otherwise it returns false.

The `hashCode`, `equals` and `toString` methods are defined for `VMID`s. Two VMIDs are considered equal if they have the same contents.

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