How do I exclude all instances of a transitive dependency when using Gradle?

Ah, the following works and does what I want:

configurations {
  runtime.exclude group: "org.slf4j", module: "slf4j-log4j12"
}

It seems that an Exclude Rule only has two attributes - group and module.
Hence for excluding from only an individual dependency, we can do something like:

dependencies {
  compile ('org.springframework.data:spring-data-hadoop-core:2.0.0.M4-hadoop22') {
    exclude group: "org.slf4j", module: "slf4j-log4j12"
  }
}

---

However, the above syntax doesn't prevent you from specifying any arbitrary property as a predicate. When trying to exclude from an individual dependency you cannot specify arbitrary properties. For example, this fails:

dependencies {
  compile ('org.springframework.data:spring-data-hadoop-core:2.0.0.M4-hadoop22') {
    exclude group: "org.slf4j", name: "slf4j-log4j12"
  }
}

with

No such property: name for class: org.gradle.api.internal.artifacts.DefaultExcludeRule

So even though you can specify a dependency with a group: and name: you can't specify an exclusion with a name:!?!

Perhaps a separate question, but what exactly is a module then? I can understand the Maven notion of groupId:artifactId:version, which I understand translates to group:name:version in Gradle. But then, how do I know what module (in gradle-speak) a particular Maven artifact belongs to?

Your approach is correct. (Depending on the circumstances, you might want to use configurations.all { exclude ... }.) If these excludes really exclude more than a single dependency (I haven't ever noticed that when using them), please file a bug at http://forums.gradle.org, ideally with a reproducible example.

For excluding one or more library globally add the following to your build.gradle

configurations.all {
   exclude group:"org.apache.geronimo.specs", module: "geronimo-servlet_2.5_spec"
   exclude group:"ch.qos.logback", module:"logback-core"
}

Now the exclude block has two properties group and module. For those of you coming from maven background, group is same as groupId and module is same as artifactId. Example: To exclude com.mchange:c3p0:0.9.2.1 following should be exclude block

exclude group:"com.mchange", module:"c3p0"