Maven does not automatically download dependencies - eclipse

Question

I'm a starter for Maven. I pulled a java maven project from Bitbucket. When compiling the prject Eclipse said "8/6/12 1:39:05 PM EST: Missing artifact com.tinkerpop.blueprints:blueprints-core:jar:1.0:compile":
Maven console:
8/6/12 1:39:03 PM EST: Missing artifact com.tinkerpop.blueprints:blueprints-core:jar:1.0:compile
8/6/12 1:39:05 PM EST: Missing artifact com.tinkerpop.blueprints:blueprints-core:jar:1.0:compile
but when I check the pom.xml file, it is already there in the dependency list, only the Type and Scope fields are empty. Seeing the errors, I try to add this dependency by myself. And it showed like this:
The #1 question is, Why for 1.0 version, blueprints-core only have a .pom file not a .jar file? Does this mean in the remote repository they don't provide the 1.0 version any more? (I found the codes in the project reply on 1.0 version instead of 2.0 version of this Blueprint framework)
The #2 questions is, under what circumstances Maven will not automatically download the jar files for the dependencies added?
This is the original pom.xml file:
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<parent>
<groupId>edu.qut.cs</groupId>
<artifactId>qut-recommender</artifactId>
<version>1.0-SNAPSHOT</version>
<relativePath>../qut-recommender/pom.xml</relativePath>
</parent>
<modelVersion>4.0.0</modelVersion>
<artifactId>rcmd-common</artifactId>
<packaging>jar</packaging>
<name>QUT Recommender Common Library and Utilty</name>
<dependencies>
<dependency>
<groupId>com.google.guava</groupId>
<artifactId>guava</artifactId>
<version>11.0.1</version>
</dependency>
<dependency>
<groupId>org.mongodb</groupId>
<artifactId>mongo-java-driver</artifactId>
<version>2.7.2</version>
<scope>compile</scope>
</dependency>
<dependency>
<groupId>org.apache.mahout</groupId>
<artifactId>mahout-ext</artifactId>
<version>1.0-SNAPSHOT</version>
<scope>compile</scope>
</dependency>
<dependency>
<groupId>jgrapht</groupId>
<artifactId>jgrapht</artifactId>
<version>0.7.3</version>
<scope>compile</scope>
</dependency>
<dependency>
<groupId>com.tinkerpop.blueprints</groupId>
<artifactId>blueprints-core</artifactId>
<version>1.0</version>
</dependency>
<dependency>
<groupId>log4j</groupId>
<artifactId>log4j</artifactId>
<version>1.2.16</version>
</dependency>
</dependencies>
</project>
The POM and Jar files of 1.0 version exist. The following picture is the directories and files in the folder of "C:\Users\n8275441.m2\repository\com\tinkerpop\blueprints". BTW, I pulled this project from Bitbucket repository. Is it possible that those 1.0 version files came along with the project in Bitbuckdet?

Answer 1

Based on your information and after checking maven central the problem is based on the version you have selected. Maven Central contains only a version 1.2, 2.0.0 and 2.1.0 but not a version 1.0.
Furthermore if a dependency is given without scope like in your pom:
<dependency>
<groupId>com.tinkerpop.blueprints</groupId>
<artifactId>blueprints-core</artifactId>
<version>1.0</version>
</dependency>
this means default scope which is "compile" (as in the sense of convention over configuration). I would simply suggest to change the version to 2.0.0 or 2.1.0 which should solve the problem.
If you check compiling such project i would suggest to do the first try on command line and not in Eclipse.

Answer 2

If you're wondering what Maven Central offers, mvnrepository.com is your friend.
In this case, there is no 1.0 version: http://mvnrepository.com/artifact/com.tinkerpop.blueprints/blueprints-core
Which leads to the interesting question why m2e reports a POM file for it. I suggest to have a look on your hard disk if the POM file really exists. You can find it (or not) in C:\Users\n8275441\.m2\repository\com\tinkerpop\blueprints\1.0\blueprints-core\
As for question #2: Maven (and m2e as well), will try to download a release (i.e. something that doesn't have SNAPSHOT in the version) only once. If the download fails, Maven will note that and never try again since it assumes that the upstream repositories (Maven Central, for example) heed the Maven rules (so if a repo doesn't have something, it won't have it tomorrow).
For snapshots, Maven will try to download them once per day to get a good balance between keeping you up to date and not wasting your time waiting for some download.