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JDBM provides TreeMap, HashMap and other collections backed up by disk storage. Now you can handle billions of items without ever running out of memory. JDBM is probably the fastest and the simpliest pure Java database.

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package org.apache.jdbm;

import java.io.*;
import java.nio.ByteBuffer;
import java.util.zip.ZipEntry;
import java.util.zip.ZipFile;

/**
 * A read-only storage which reads data from compressed zip archive.
 * 
 * To improve performance with compressed archives
 * each page is stored in separate file (zip archive entry).
 */
class StorageZip implements Storage {


    private String zip;
    private String zip2;
    private ZipFile z;

    StorageZip(String zipFile) throws IOException {
        zip = zipFile;
        z = new ZipFile(zip);
        zip2 = "db";
    }

    public void write(long pageNumber, ByteBuffer data) throws IOException {
        throw new UnsupportedOperationException("readonly");
    }

    public ByteBuffer read(long pageNumber) throws IOException {
        ByteBuffer data = ByteBuffer.allocate(PAGE_SIZE);

        ZipEntry e = z.getEntry(zip2 + pageNumber);
        if(e == null)
            return ByteBuffer.wrap(PageFile.CLEAN_DATA).asReadOnlyBuffer();

        InputStream i = z.getInputStream(e);
        new DataInputStream(i).readFully(data.array());
        i.close();
        return data;
    }

    public void forceClose() throws IOException {
        z.close();
        z = null;
    }

    public DataInputStream readTransactionLog() {
        throw new UnsupportedOperationException("readonly");
    }

    public void deleteTransactionLog() {
        throw new UnsupportedOperationException("readonly");
    }

    public void sync() throws IOException {
        throw new UnsupportedOperationException("readonly");
    }

    public DataOutputStream openTransactionLog() throws IOException {
        throw new UnsupportedOperationException("readonly");
    }

    public void deleteAllFiles() throws IOException {
    }

    public boolean isReadonly() {
        return true;
    }
}