For testing the read performance of the MongoDB store with real world data, the database content of a Tricia installation with approximately 440.000 rows was used. The test itself consisted of 20 simulated users who vised the start page, some wikis and group pages simultaneously. Each test run made 1600 requests to the Tricia instance and was performed on a virtual machine with Windows Vista 64 Bit on an Intel Xeon processor with 2,5GHz and 1,5GB of RAM.
Table 7.2 shows some of the results of this test. MongoDB performed most queries faster than any of the two other databases. The query with the slowest average time to MongoDB took a little bit longer than the slowest query in MySQL, how- ever MongoDB performed the next slowest queries a lot faster than MySQL. In total MongoDB was more than three times quicker as MySQL and more than two times faster than HSQL, despite that the join operations were not performed in- side the database. The results of the MonitorStore showed that the join operations performed by the MongoQuery class did not have a noticeable impact on the per- formance.
The total times in table 7.2 show how much time was spent for the ve database operations, which consumed the most time during the test.
MySQL HSQL MongoDB
Total time spent
for db operations 3375s 2315s 948s
Average times of the ve slowest operations in ms
1140, 890, 775,
763, 578 1461, 777, 648,547, 491 1306, 208, 142,139, 109 The ve greatest
total times in s 1137, 494, 404,297, 136 970, 270, 209, 186,121 433, 150, 105, 90,56 Table 7.2: Performance results with real world data.
8 Conclusion
This work has shown that there are a lot of dierences between current NoSQL implementations. This is not a surprising result, because none of the examined databases aim to be the perfect solution for every problem. Instead each database has its own properties and features, so that developers have to choose the right database depended on the requirements of their project. Therefore this work com- pared the trade-os of the examined database implementations and explained why these are necessary.
Furthermore was an Object-relational Persistence Layer based on a NoSQL database for a Web Collaboration and Knowledge Management Software successfully imple- mented. This showed that NoSQL databases can ful ll the requirements of such an application. It has also shown that the usage of a NoSQL database can increase the performance signi cantly, even on a single machine. Aside from this was the development process for the replacement of a persistence layer of an already existing application outlined.
Using MongoDB for the persistence layer also has some disadvantages compared to established relational databases like MySQL. The biggest problem with MongoDB is the durability. By using memory mapped les, MongoDB becomes very vulner- able to power failures and other kinds of system failures [6].
The other problem is the administration, because MongoDB has no built in graph- ical con guration tools or graphical query browsers yet. There are some projects which attempt to provide graphical user interfaces for the administration of Mon- goDB databases [1], but they are currently not in a state comparable to the ones of the already established databases.
The prototype implementation instruments join operations that are performed by the client to realize the relations between the entities. Even though this imple- mentation was faster then the counterparts for MySQL, the performance could be even more increased through the usage of nested documents in MongoDB. But this would have required more fundamental changes in the architecture of the persistence layer.
8. Conclusion
Another interesting topic for future works could be the implementation of support layers for other NoSQL databases for Tricia, which would give the opportunity to compare the performance of them with real world data.
A Listing of MongoQuery.java
package de . i n f o a s s e t . platform . s t o r e . mongodb ;
import java . u t i l . HashMap ;
import java . u t i l . I t e r a t o r ;
5 import java . u t i l . L i s t ;
import com . google . common . c o l l e c t . Maps ;
import com . mongodb . BasicDBList ;
import com . mongodb . BasicDBObject ;
10 import com . mongodb . DBCursor ;
import de . i n f o a s s e t . platform . s e r v i c e s . Loggers ;
import de . i n f o a s s e t . platform . s t o r e . AbstractAttribute ;
import de . i n f o a s s e t . platform . s t o r e . AbstractQuery ;
15 import de . i n f o a s s e t . platform . s t o r e . AbstractQueryVisitor ;
import de . i n f o a s s e t . platform . s t o r e . Container ;
import de . i n f o a s s e t . platform . s t o r e . Content ;
import de . i n f o a s s e t . platform . s t o r e . Join ;
import de . i n f o a s s e t . platform . s t o r e . Query ;
20 import de . i n f o a s s e t . platform . s t o r e . QueryCondition ;
import de . i n f o a s s e t . platform . s t o r e . QueryEquals ;
import de . i n f o a s s e t . platform . s t o r e . QueryGreater ;
import de . i n f o a s s e t . platform . s t o r e . QueryGreaterOrEqual ;
import de . i n f o a s s e t . platform . s t o r e . QueryLess ;
25 import de . i n f o a s s e t . platform . s t o r e . QueryLessOrEqual ;
import de . i n f o a s s e t . platform . s t o r e . QueryNot ;
import de . i n f o a s s e t . platform . s t o r e . S o r t i n g C r i t e r i o n ;
p u b l i c c l a s s MongoQuery {
30
/∗∗ The Container a g a i n s t t h i s query i s made . ∗/ MongoContainer c o n t a i n e r ;
/∗∗ The query as mongodb o b j e c t . ∗/
35 BasicDBObject mongoQuery = new BasicDBObject ( ) ;
/∗∗ The second part o f the query , i f the query c o n t a i n s a j o i n operation . ∗/
A. Listing of MongoQuery.java
boolean j o i n = f a l s e ;
/∗ The c o n t a i n e r that i s used f o r the joinedQuery . ∗/ MongoContainer jo i n C o n ta in e r ;
45
/∗
∗ The name o f the a t t r i b u t e which i s used to j o i n the q u e r i e s on the l e f t
∗ s i d e o f the query . ∗/
50 S t r i n g l e f t J o i n A t t r i b u t e ;
/∗
∗ The name o f the a t t r i b u t e which i s used to j o i n the q u e r i e s on the s i d e
∗ o f the j o i n e d query .
55 ∗/
S t r i n g r i g h t J o i n A t t r i b u t e ;
/∗ The o r d e r i n g o f the query . ∗/
BasicDBObject orderBy = new BasicDBObject ( ) ;
60
/∗ The o r d e r i n g o f the j o i n part o f the query . ∗/
BasicDBObject joinOrderBy = new BasicDBObject ( ) ;
p u b l i c MongoQuery ( MongoContainer mongoContainer , AbstractQuery q ,
65 List <String > types ) {
t h i s. c o n t a i n e r = mongoContainer ; buildQuery (q , types ) ;
}
70 /∗ Analyzes q u e r i e s and prepares them f o r mongodb . ∗/
p r i v a t e void buildQuery ( AbstractQuery q , List <String > types ) {
i f ( ! ( q i n s t a n c e o f Query ) ) {
i f ( Loggers . mongoLog . isDebugEnabled ( ) ) {
Loggers . mongoLog . debug ( " unsupported query type "
75 + q . g e t C la s s ( ) ) ;
} }
Query query = ( Query ) q ;
i f ( Loggers . mongoLog . isDebugEnabled ( ) ) {
80 Loggers . mongoLog . debug ( " buildQuery f o r : " + query
+ " in " + c o n t a i n e r . getTableName ( ) ) ; }
// Create type c o n d i t i o n s
85 i f ( types != n u l l) {
i f ( types . s i z e ( ) == 1) {
mongoQuery . put ( " type " , types . get ( 0 ) ) ; } e l s e i f ( types . s i z e ( ) > 1) {
// use the $in operator f o r m u l t i p l e types
A. Listing of MongoQuery.java
90 BasicDBList t y p e L i s t = new BasicDBList ( ) ;
i n t i = 0 ;
f o r ( S t r i n g type : types ) { t y p e L i s t . put ( i ++,type ) ; }
95 mongoQuery . put ( " type " ,new BasicDBObject ( " $in " ,
t y p e L i s t ) ) ; }
}
100 // v i s i t a l l parts o f the query
query . v i s i t (new AbstractQueryVisitor ( ) {
// t h i s map s t o r e s a l l already v i s i t e d equals c o n d i t i o n s HashMap<QueryEquals , QueryEquals> used = Maps
. newHashMap ( ) ;
105
@Override
protected void visitOp ( Query q ) {
i f ( used . containsKey ( q ) )
// do not look again on an already v i s i t e d query
110 return;
// handle j o i n o p e r a t i o n s
f o r ( Join j : q . g e t J o i n s ( ) ) {
Container cont1 = castToMongoContainer ( j . g e t F i r s t A t t r i b u t e S i g n a t u r e s ( )
115 . getContainer ( ) ) ;
Container cont2 = castToMongoContainer ( j . getSecondAttributeSignatures ( ) . getContainer ( ) ) ;
i f ( j oin C o n t a i ne r != n u l l) {
120 i f ( Loggers . mongoLog . isDebugEnabled ( ) ) {
Loggers . mongoLog
. debug ( " Queries with more than one j o i n operation are not
supported ! " ) ; } Thread . dumpStack ( ) ; 125 i f ( MongoContainer . stopOnError ( ) ) System . e x i t (−1) ; } i f ( cont1 != c o n t a i n e r ) { 130 jo in C o nt a in e r = castToMongoContainer ( cont1 ) ; l e f t J o i n A t t r i b u t e = AbstractAttribute . getColumnName ( j . getSecondAttributeSignatures ( ) ) ; 135 r i g h t J o i n A t t r i b u t e = AbstractAttribute . getColumnName ( j . g e t F i r s t A t t r i b u t e S i g n a t u r e s ( ) ) ;
A. Listing of MongoQuery.java jo in C o nt a in e r = castToMongoContainer ( cont2 ) ; 140 r i g h t J o i n A t t r i b u t e = AbstractAttribute . getColumnName ( j . getSecondAttributeSignatures ( ) ) ; l e f t J o i n A t t r i b u t e = AbstractAttribute . getColumnName ( j 145 . g e t F i r s t A t t r i b u t e S i g n a t u r e s ( ) ) ; }
// convert the id names to MongoDB id names
i f ( r i g h t J o i n A t t r i b u t e . equals ( " id " ) ) r i g h t J o i n A t t r i b u t e = "_id" ;
150 i f ( l e f t J o i n A t t r i b u t e . equals ( " id " ) )
l e f t J o i n A t t r i b u t e = "_id" ;
// t h i s query obviously has a j o i n operation j o i n = true;
}
155 // currentQuerry p o i n t s e i t h e r to mongoQuerry or to
the
// joinedQuerry
// depending on the c o n t a i n e r o f the c o n d i t i o n BasicDBObject currentQuery = mongoQuery ;
i f ( q i n s t a n c e o f QueryCondition ) {
160 QueryCondition c = ( QueryCondition ) q ;
Container cont = c . g e t A t t r i b u t e S i g n a t u r e ( ) . getContainer ( ) ;
165 i f ( ! cont . getTableName ( ) . equals (
c o n t a i n e r . getTableName ( ) ) ) { j o i n = true;
// t h i s c o n d i t i o n belongs to the joinedQuerry currentQuery = joinedQuery ; 170 } // t r a n s l a t e the d i f f e r e n t types o f c o n d i t i o n s i n t o MongoDB // c o n d i t i o n s i f ( q i n s t a n c e o f QueryEquals ) { 175 QueryEquals qe = ( QueryEquals ) q ;
currentQuery . put ( AbstractAttribute . getColumnName ( qe
. g e t A t t r i b u t e S i g n a t u r e ( ) ) , qe . getValue ( ) ) ;
180 } e l s e i f ( q i n s t a n c e o f QueryGreater ) {
QueryGreater qr = ( QueryGreater ) q ; BasicDBObject b = new BasicDBObject ( ) ; b . put ( AbstractAttribute . getColumnName ( qr
. g e t A t t r i b u t e S i g n a t u r e ( ) ) , qr
185 . getValue ( ) ) ;
currentQuery . put ( "> " ,b) ;
} e l s e i f ( q i n s t a n c e o f QueryGreaterOrEqual ) {
A. Listing of MongoQuery.java
QueryGreaterOrEqual qr = ( QueryGreaterOrEqual ) q ;
BasicDBObject b = new BasicDBObject ( ) ;
190 b . put ( AbstractAttribute . getColumnName ( qr
. g e t A t t r i b u t e S i g n a t u r e ( ) ) , qr . getValue ( ) ) ;
currentQuery . put ( ">e " ,b) ; } e l s e i f ( q i n s t a n c e o f QueryLess ) {
195 QueryLess qr = ( QueryLess ) q ;
BasicDBObject b = new BasicDBObject ( ) ; b . put ( AbstractAttribute . getColumnName ( qr
. g e t A t t r i b u t e S i g n a t u r e ( ) ) , qr . getValue ( ) ) ;
200 currentQuery . put ( "&l t " , b) ;
} e l s e i f ( q i n s t a n c e o f QueryLessOrEqual ) { QueryLessOrEqual qr = ( QueryLessOrEqual ) q ; BasicDBObject b = new BasicDBObject ( ) ; b . put ( AbstractAttribute . getColumnName ( qr
205 . g e t A t t r i b u t e S i g n a t u r e ( ) ) , qr
. getValue ( ) ) ;
currentQuery . put ( "&l t e " ,b) ; } e l s e {
i f ( Loggers . mongoLog . isDebugEnabled ( ) ) {
210 Loggers . mongoLog . e r r o r ( " c o n d i t i o n type "
+ q . g e t C la s s ( )
+ " i s not supported " ) ; }
}
215 } e l s e i f ( q i n s t a n c e o f QueryNot ) {
// handle not operations , which are only supported // i f they are used to determine i f an a t t r i b u t e
i s s e t
// and can t h e r e f o r e be transformed i n t o an $ e x i s t s
// c o n d i t i o n
220 QueryNot qn = ( QueryNot ) q ;
i f ( qn . getTerm ( ) i n s t a n c e o f QueryEquals ) {
QueryEquals qe = ( QueryEquals ) qn . getTerm ( ) ; currentQuery . put ( AbstractAttribute
. getColumnName ( qe
225 . g e t A t t r i b u t e S i g n a t u r e ( ) ) ,
new BasicDBObject ( " $ e x i s t s " ,true) ) ; used . put ( qe , qe ) ;
} e l s e {
Loggers . mongoLog
230 . e r r o r ( " cannot p r o c e s s not operator " ) ;
} } }
A. Listing of MongoQuery.java
// Handle the s o r t order :
f o r ( S o r t i n g C r i t e r i o n sc : query . g e t A l l S o r t i n g C r i t e r i o n s ( ) ) { S t r i n g name = AbstractAttribute . getColumnName ( sc
240 . g e t A t t r i b u t e S i g n a t u r e ( ) ) ;
// convert id a t t r i b u t e name to MongoDB
i f (name . equals ( " id " ) ) name = "_id" ;
245 i f ( ! sc . g e t A t t r i b u t e S i g n a t u r e ( ) . getContainer ( )
. getTableName ( ) . equals ( c o n t a i n e r . getTableName ( ) ) ) {
// t h i s o r d e r i n g i n s t r u c t i o n i s f o r the joinedQuery
i f ( sc . isAscending ( ) ) { joinOrderBy . put (name , 1 ) ;
250 } e l s e {
joinOrderBy . put (name, −1) ; }
} e l s e {
// t h i s one f o r the mongoQuery
255 i f ( sc . isAscending ( ) ) {
orderBy . put (name , 1 ) ; } e l s e {
orderBy . put (name, −1) ; }
260 i f ( j o i n ) {
Loggers . mongoLog
. e r r o r ( " Sorting in the second part o f a j o i n e d query i s not supported " ) ; }
}
265 }
// some debug code :
i f ( Loggers . mongoLog . isDebugEnabled ( ) ) {
Loggers . mongoLog . debug ( " created mongoQuery : "
270 + mongoQuery ) ;
i f ( j o i n ) {
Loggers . mongoLog . debug ( " created joinedQuery : " + joinedQuery ) ;
Loggers . mongoLog . debug ( " j o i n l e f t : "
275 + l e f t J o i n A t t r i b u t e + " r i g h t : "
+ r i g h t J o i n A t t r i b u t e ) ;
Loggers . mongoLog . debug ( " orderBy : " + orderBy ) ; }
}
280 }
/∗
∗ @return An i t e r a t o r over the content o b j e c t s in the r e s u l t s e t o f t h i s
A. Listing of MongoQuery.java
∗ query
285 ∗/
p u b l i c I t e r a t o r <Content> g e t C o n t e n t I t e r a t o r (i n t l i m i t ) {
i f ( j o i n ) {
return new Con t e nt It er a t or (new J o i n I t e r a t o r ( l i m i t ) ) ; } e l s e { 290 DBCursor c u r s o r = c o n t a i n e r . g e t C o l l e c t i o n ( ) . f i n d ( mongoQuery ) ; i f ( l i m i t > 0) c u r s o r . l i m i t ( l i m i t + 1) ; c u r s o r . s o r t ( orderBy ) ;
295 return new Con t e nt It er a t or (new S i m p l e I t e r a t o r ( c u r s o r ) ) ;
} }
/∗ @return An i t e r a t o r o f the i d e n t e n t i f i e r s o f the query r e s u l t s . ∗/
300 p u b l i c I t e r a t o r <String > g e t I d I t e r a t o r (i n t l i m i t ) {
i f ( j o i n ) {
return new I d I t e r a t o r (new J o i n I t e r a t o r ( l i m i t ) ) ; } e l s e { DBCursor c u r s o r = c o n t a i n e r . g e t C o l l e c t i o n ( ) . f i n d ( 305 mongoQuery ) ; i f ( l i m i t > 0) c u r s o r . l i m i t ( l i m i t + 1) ; c u r s o r . s o r t ( orderBy ) ;
return new I d I t e r a t o r (new S i m p l e I t e r a t o r ( c u r s o r ) ) ;
310 }
}
/∗ Del e t es a l l o b j e c t s in the database that are in the query r e s u l t s e t . ∗/
p u b l i c void d e l e t e ( ) {
315 i f ( j o i n ) {
DBCursor c u r s o r = j oin C on t ain e r . g e t C o l l e c t i o n ( ) . f i n d ( joinedQuery ) ;
while ( c u r s o r . hasNext ( ) ) {
BasicDBObject obj = ( BasicDBObject ) c u r s o r . next ( ) ;
320 mongoQuery . put ( l e f t J o i n A t t r i b u t e , obj
. get ( r i g h t J o i n A t t r i b u t e ) ) ; try { c o n t a i n e r . g e t C o l l e c t i o n ( ) . remove ( mongoQuery ) ; } catch ( Exception ex ) { 325 ex . printStackTrace ( ) ; } } } e l s e { 330 c o n t a i n e r . g e t C o l l e c t i o n ( ) . remove ( mongoQuery ) ; }
A. Listing of MongoQuery.java
p u b l i c MongoContainer castToMongoContainer ( Container cont ) {
335 i f ( cont i n s t a n c e o f MongoContainer ) {
return ( MongoContainer ) cont ; } e l s e {
return ( MongoContainer ) c o n t a i n e r . g e t S t o r e ( ) . getContainer ( cont . getTableName ( ) ) ;
340 }
}
/∗ @return The s i z e o f the r e s u l t s e t o f t h i s query . ∗/
345 p u b l i c i n t count ( ) {
i f ( j o i n ) {
DBCursor c u r s o r = j oin C on t ain e r . g e t C o l l e c t i o n ( ) . f i n d ( joinedQuery ) ;
i n t sum = 0 ;
350 HashMap<String , String > used = Maps . newHashMap ( ) ;
while ( c u r s o r . hasNext ( ) ) {
BasicDBObject obj = ( BasicDBObject ) c u r s o r . next ( ) ; S t r i n g id = obj . g e t S t r i n g ( "_id" ) . t o S t r i n g ( ) ;
i f ( ! used . containsKey ( id ) ) {
355 mongoQuery . put ( l e f t J o i n A t t r i b u t e , obj
. get ( r i g h t J o i n A t t r i b u t e ) ) ; sum += c o n t a i n e r . g e t C o l l e c t i o n ( ) . f i n d ( mongoQuery ) . count ( ) ; } 360 } return sum ; } e l s e {
return c o n t a i n e r . g e t C o l l e c t i o n ( ) . f i n d ( mongoQuery ) . count ( ) ; } 365 } /∗ An I t e r a t o r f o r q u e r i e s without j o i n s . ∗/ p r i v a t e c l a s s S i m p l e I t e r a t o r implements I t e r a t o r <BasicDBObject> { DBCursor c u r s o r ; 370 p u b l i c S i m p l e I t e r a t o r ( DBCursor c u r s o r ) { t h i s. c u r s o r = c u r s o r ; } 375 @Override p u b l i c boolean hasNext ( ) { return c u r s o r . hasNext ( ) ; } 380 @Override p u b l i c BasicDBObject next ( ) {
BasicDBObject r e s u l t = ( BasicDBObject ) c u r s o r . next ( ) ;
return r e s u l t ;
A. Listing of MongoQuery.java } 385 @Override p u b l i c void remove ( ) { c u r s o r . remove ( ) ; } 390 }
/∗ This I t e r a t o r i s used f o r q u e r i e s with a j o i n operation . ∗/
p r i v a t e c l a s s J o i n I t e r a t o r implements I t e r a t o r <BasicDBObject> {
395 /∗ Cursor over the r e s u l t s o f the joinedQuery . ∗/
DBCursor joinedCursor ;
/∗ Cursor over the f i n a l r e s u l t s e t s . ∗/ DBCursor c u r s o r ;
400
/∗ The o b j e c t that was l a s t returned by next ( ) . ∗/ BasicDBObject current ;
/∗ The next o b j e c t that w i l l be returned by next ( ) . ∗/
405 BasicDBObject next ;
i n t l i m i t ;
i n t n = 0 ;
410
/∗ Already used o b j e c t s from the joinedCursor . ∗/ HashMap<String , String > used = Maps . newHashMap ( ) ;
p u b l i c J o i n I t e r a t o r (i n t l i m i t ) {
415 i f ( j oin C o n t ain er == n u l l) {
Loggers . mongoLog . e r r o r ( " j o in C o n t a i ne r n u l l " ) ; }
// i n i t the c u r s o r o f the joinedQuery
joinedCursor = j o inC o n t a in e r . g e t C o l l e c t i o n ( ) . f i n d (
420 joinedQuery ) ;
// some debug code
i f ( Loggers . mongoLog . isDebugEnabled ( ) ) {
Loggers . mongoLog . debug ( " J o i n I t e r a t o r : c u r s o r " + joinedQuery + " cursorcount : "
425 + joinedCursor . count ( ) ) ;
Loggers . mongoLog . debug ( " J o i n I t e r a t o r : c u r s o r on " + j o in C o n t a i ne r . g e t C o l l e c t i o n ( ) . getName ( ) ) ; Loggers . mongoLog . debug ( " J o i n I t e r a t o r : c u r s o r c o l l e c t i o n s i z e " 430 + j o in C o n t a i n e r . g e t C o l l e c t i o n ( ) . getCount ( ) ) ; }
A. Listing of MongoQuery.java
435 /∗ Find the f i r s t element . ∗/
searchNext ( ) ; t h i s. l i m i t = l i m i t ; } 440 @Override p u b l i c boolean hasNext ( ) { return next != n u l l; } 445 @Override p u b l i c BasicDBObject next ( ) { current = next ;
searchNext ( ) ; // the i t e r a t o r i s always a step ahead to determine i f
// th ere i s a next element
450 n++;
return current ; }
/∗
455 ∗ Find the next element . In order to do t hi s , t h i s method has
to
∗ perform the j o i n operation . ∗/
p r i v a t e void searchNext ( ) { next = n u l l;
460 i f ( c u r s o r == n u l l | | ! c u r s o r . hasNext ( ) ) {
// i f the c u r s o r cannot return anymore elements , then increment
// the joinedCursor
// u n t i l the joinedCursor has reached i t s end or a new c u r s o r i s
// found with
465 // more r e s u l t elements
while ( joinedCursor . hasNext ( )
&& ( c u r s o r == n u l l | | ! c u r s o r . hasNext ( ) ) ) { S t r i n g value = joinedCursor . next ( ) . get (
r i g h t J o i n A t t r i b u t e ) . t o S t r i n g ( ) ;
470 i f ( ! used . containsKey ( value ) ) {
used . put ( value , value ) ;
mongoQuery . put ( l e f t J o i n A t t r i b u t e , value ) ;
i f ( Loggers . mongoLog . isDebugEnabled ( ) ) { Loggers . mongoLog 475 . debug ( " J o i n I t e r a t o r : c u r s o r " + mongoQuery ) ; } c u r s o r = c o n t a i n e r . g e t C o l l e c t i o n ( ) . f i n d ( mongoQuery ) ; 480 c u r s o r . l i m i t ( l i m i t ) ; } 87
A. Listing of MongoQuery.java
} }
// i f the c u r s o r has another element , then get i t
485 i f ( c u r s o r != n u l l && c u r s o r . hasNext ( ) ) {
next = ( BasicDBObject ) c u r s o r . next ( ) ;
i f ( Loggers . mongoLog . isDebugEnabled ( ) ) {
Loggers . mongoLog . debug ( " J o i n I t e r a t o r : next " + next ) ; 490 } } } 495 @Override p u b l i c void remove ( ) {
c o n t a i n e r . deleteContent ( current . get ( "_id" ) . t o S t r i n g ( ) ) ; } } 500 /∗ An I t e r a t o r that r e t u r n s Content o b j e c t s . ∗/ p r i v a t e c l a s s Co n t en tI te ra to r implements I t e r a t o r <Content> { I t e r a t o r <BasicDBObject> i t e r ; 505 p u b l i c C o nt e nt I t er at o r ( I t e r a t o r <BasicDBObject> i t e r ) { t h i s. i t e r = i t e r ; } 510 @Override p u b l i c boolean hasNext ( ) { return i t e r . hasNext ( ) ; } 515 @Override p u b l i c Content next ( ) {
MongoContent content = c o n t a i n e r . contentFactory ( ) ; BasicDBObject dbObject = i t e r . next ( ) ;
content . i n i t i a l i z e ( container , dbObject . get ( "_id" )
520 . t o S t r i n g ( ) ,n u l l) ;
content . read ( dbObject ) ;
return content ; } 525 @Override p u b l i c void remove ( ) { i t e r . remove ( ) ; } } 530
A. Listing of MongoQuery.java p r i v a t e c l a s s I d I t e r a t o r implements I t e r a t o r <String > { I t e r a t o r <BasicDBObject> i t e r ; 535 p u b l i c I d I t e r a t o r ( I t e r a t o r <BasicDBObject> i t e r ) { t h i s. i t e r = i t e r ; } 540 @Override p u b l i c boolean hasNext ( ) { return i t e r . hasNext ( ) ; } 545 @Override p u b l i c S t r i n g next ( ) {
return i t e r . next ( ) . get ( "_id" ) . t o S t r i n g ( ) ; } 550 @Override p u b l i c void remove ( ) { i t e r . remove ( ) ; } } 555 } 89
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