If the generator hasn't got one cached, it needs to go to the database in Java

Generation QR Code JIS X 0510 in Java If the generator hasn't got one cached, it needs to go to the database
If the generator hasn't got one cached, it needs to go to the database
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class KeyGenerator private void reserveIds() { PreparedStatement stmt = null; ResultSet rs = null; long newNextId; try { stmt = connprepareStatement("SELECT nextID FROM keys WHERE name = FOR UPDATE"); stmtsetString(1, keyName); rs = stmtexecuteQuery(); rsnext(); newNextId = rsgetLong(1); } catch (SQLException exc) { throw new ApplicationException("Unable to generate ids", exc); } finally { DBcleanUp(stmt, rs); } long newMaxId = newNextId + incrementBy; stmt = null;
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try { stmt = connprepareStatement("UPDATE keys SET nextID = WHERE name = "); stmtsetLong(1, newMaxId); stmtsetString(2, keyName); stmtexecuteUpdate(); conncommit(); nextId = newNextId; maxId = newMaxId; } catch (SQLException exc) { throw new ApplicationException("Unable to generate ids", exc); } finally { DBcleanUp(stmt); } }
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In this case we use SELECT FOR UPDATE to tell the database to hold a write lock on the key table This is an Oracle-specific statement, so your mileage will vary if you're using something else If you can't write-lock on the select, you run the risk of the transaction failing should another one get in there before you In this case, however, you can pretty safely just rerun reserveIds until you get a pristine set of keys
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Example: Using a Compound Key (Java)
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Using a simple integral key is a good, simple solution, but you often need other types or compound keys A Key Class
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As soon as you need something else it's worth putting together a key class A key class needs to be able to store multiple elements of the key and to be able to tell if two keys are equal
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class Key private Object[] fields; public boolean equals(Object obj) { if (!(obj instanceof Key)) return false; Key otherKey = (Key) obj; if (thisfieldslength != otherKeyfieldslength) return false; for (int i = 0; i < fieldslength; i++) if (!thisfields[i]equals(otherKeyfields[i])) return false; return true; }
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The most elemental way to create a key is with an array parameter
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class Key public Key(Object[] fields) { checkKeyNotNull(fields); thisfields = fields; } private void checkKeyNotNull(Object[] fields) { if (fields == null) throw new IllegalArgumentException("Cannot have a null key"); for (int i = 0; i < fieldslength; i++) if (fields[i] == null) throw new IllegalArgumentException("Cannot have a null element of key"); }
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If you find you commonly create keys with certain elements, you can add convenience constructors The exact ones will depend on what kinds of keys your application has
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class Key public Key(long arg) { thisfields = new Object[1]; thisfields[0] = new Long(arg); } public Key(Object field) { if (field == null) throw new IllegalArgumentException("Cannot have a null key"); thisfields = new Object[1]; thisfields[0] = field; } public Key(Object arg1, Object arg2) { thisfields = new Object[2]; thisfields[0] = arg1; thisfields[1] = arg2; checkKeyNotNull(fields); }
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Don't be afraid to add these convenience methods After all, convenience is important to everyone using the keys
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Similarly you can add accessor functions to get parts of keys The application will need to do this for the mappings
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class Key public Object value(int i) { return fields[i]; } public Object value() { checkSingleKey(); return fields[0]; } private void checkSingleKey() { if (fieldslength > 1) throw new IllegalStateException("Cannot take value on composite key"); } public long longValue() { checkSingleKey(); return longValue(0); } public long longValue(int i) { if (!(fields[i] instanceof Long)) throw new IllegalStateException("Cannot take longValue on non long key"); return ((Long) fields[i])longValue(); }
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In this example we'll map to an order and line item tables The order table has a simple integral primary key, the line item table's primary key is a compound of the order's primary key and a sequence number
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CREATE TABLE orders (ID int primary key, customer varchar) CREATE TABLE line_items (orderID int, seq int, amount int, product varchar, primary key (orderID, seq))
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The Layer Supertype (475) for domain objects needs to have a key field
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class DomainObjectWithKey
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private Key key; protected DomainObjectWithKey(Key ID) { thiskey = ID; } protected DomainObjectWithKey() { } public Key getKey() { return key; } public void setKey(Key key) { thiskey = key; }
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