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| <h1>Java 8 Programming Guide</h1> |
| |
| |
| |
| <p>Java 8 introduces several new language features designed for faster and clearer coding. With the most important feature, |
| the so-called “Lambda Expressions”, Java 8 opens the door to functional programming. Lambda Expressions allow for implementing and |
| passing functions in a straightforward way without having to declare additional (anonymous) classes.</p> |
| |
| <p>The newest version of Flink supports the usage of Lambda Expressions for all operators of the Java API. |
| This document shows how to use Lambda Expressions and describes current limitations. For a general introduction to the |
| Flink API, please refer to the <a href="programming_guide.html">Programming Guide</a></p> |
| |
| <ul id="markdown-toc"> |
| <li><a href="#examples" id="markdown-toc-examples">Examples</a></li> |
| <li><a href="#compiler-limitations" id="markdown-toc-compiler-limitations">Compiler Limitations</a></li> |
| </ul> |
| |
| <h3 id="examples">Examples</h3> |
| |
| <p>The following example illustrates how to implement a simple, inline <code>map()</code> function that squares its input using a Lambda Expression. |
| The types of input <code>i</code> and output parameters of the <code>map()</code> function need not to be declared as they are inferred by the Java 8 compiler.</p> |
| |
| <div class="highlight"><pre><code class="language-java"><span class="n">env</span><span class="o">.</span><span class="na">fromElements</span><span class="o">(</span><span class="mi">1</span><span class="o">,</span> <span class="mi">2</span><span class="o">,</span> <span class="mi">3</span><span class="o">)</span> |
| <span class="c1">// returns the squared i</span> |
| <span class="o">.</span><span class="na">map</span><span class="o">(</span><span class="n">i</span> <span class="o">-></span> <span class="n">i</span><span class="o">*</span><span class="n">i</span><span class="o">)</span> |
| <span class="o">.</span><span class="na">print</span><span class="o">();</span></code></pre></div> |
| |
| <p>The next two examples show different implementations of a function that uses a <code>Collector</code> for output. |
| Functions, such as <code>flatMap()</code>, require a output type (in this case <code>String</code>) to be defined for the <code>Collector</code> in order to be type-safe. |
| If the <code>Collector</code> type can not be inferred from the surrounding context, it need to be declared in the Lambda Expression’s parameter list manually. |
| Otherwise the output will be treated as type <code>Object</code> which can lead to undesired behaviour.</p> |
| |
| <div class="highlight"><pre><code class="language-java"><span class="n">DataSet</span><span class="o"><</span><span class="n">String</span><span class="o">></span> <span class="n">input</span> <span class="o">=</span> <span class="n">env</span><span class="o">.</span><span class="na">fromElements</span><span class="o">(</span><span class="mi">1</span><span class="o">,</span> <span class="mi">2</span><span class="o">,</span> <span class="mi">3</span><span class="o">);</span> |
| |
| <span class="c1">// collector type must be declared</span> |
| <span class="n">input</span><span class="o">.</span><span class="na">flatMap</span><span class="o">((</span><span class="n">Integer</span> <span class="n">number</span><span class="o">,</span> <span class="n">Collector</span><span class="o"><</span><span class="n">String</span><span class="o">></span> <span class="n">out</span><span class="o">)</span> <span class="o">-></span> <span class="o">{</span> |
| <span class="k">for</span><span class="o">(</span><span class="kt">int</span> <span class="n">i</span> <span class="o">=</span> <span class="mi">0</span><span class="o">;</span> <span class="n">i</span> <span class="o"><</span> <span class="n">number</span><span class="o">;</span> <span class="n">i</span><span class="o">++)</span> <span class="o">{</span> |
| <span class="n">out</span><span class="o">.</span><span class="na">collect</span><span class="o">(</span><span class="s">"a"</span><span class="o">);</span> |
| <span class="o">}</span> |
| <span class="o">})</span> |
| <span class="c1">// returns "a", "a", "aa", "a", "aa" , "aaa"</span> |
| <span class="o">.</span><span class="na">print</span><span class="o">();</span></code></pre></div> |
| |
| <div class="highlight"><pre><code class="language-java"><span class="n">DataSet</span><span class="o"><</span><span class="n">String</span><span class="o">></span> <span class="n">input</span> <span class="o">=</span> <span class="n">env</span><span class="o">.</span><span class="na">fromElements</span><span class="o">(</span><span class="mi">1</span><span class="o">,</span> <span class="mi">2</span><span class="o">,</span> <span class="mi">3</span><span class="o">);</span> |
| |
| <span class="c1">// collector type must not be declared, it is inferred from the type of the dataset</span> |
| <span class="n">DataSet</span><span class="o"><</span><span class="n">String</span><span class="o">></span> <span class="n">manyALetters</span> <span class="o">=</span> <span class="n">input</span><span class="o">.</span><span class="na">flatMap</span><span class="o">((</span><span class="n">number</span><span class="o">,</span> <span class="n">out</span><span class="o">)</span> <span class="o">-></span> <span class="o">{</span> |
| <span class="k">for</span><span class="o">(</span><span class="kt">int</span> <span class="n">i</span> <span class="o">=</span> <span class="mi">0</span><span class="o">;</span> <span class="n">i</span> <span class="o"><</span> <span class="n">number</span><span class="o">;</span> <span class="n">i</span><span class="o">++)</span> <span class="o">{</span> |
| <span class="n">out</span><span class="o">.</span><span class="na">collect</span><span class="o">(</span><span class="s">"a"</span><span class="o">);</span> |
| <span class="o">}</span> |
| <span class="o">});</span> |
| |
| <span class="c1">// returns "a", "a", "aa", "a", "aa" , "aaa"</span> |
| <span class="n">manyALetters</span><span class="o">.</span><span class="na">print</span><span class="o">();</span></code></pre></div> |
| |
| <p>The following code demonstrates a word count which makes extensive use of Lambda Expressions.</p> |
| |
| <div class="highlight"><pre><code class="language-java"><span class="n">DataSet</span><span class="o"><</span><span class="n">String</span><span class="o">></span> <span class="n">input</span> <span class="o">=</span> <span class="n">env</span><span class="o">.</span><span class="na">fromElements</span><span class="o">(</span><span class="s">"Please count"</span><span class="o">,</span> <span class="s">"the words"</span><span class="o">,</span> <span class="s">"but not this"</span><span class="o">);</span> |
| |
| <span class="c1">// filter out strings that contain "not"</span> |
| <span class="n">input</span><span class="o">.</span><span class="na">filter</span><span class="o">(</span><span class="n">line</span> <span class="o">-></span> <span class="o">!</span><span class="n">line</span><span class="o">.</span><span class="na">contains</span><span class="o">(</span><span class="s">"not"</span><span class="o">))</span> |
| <span class="c1">// split each line by space</span> |
| <span class="o">.</span><span class="na">map</span><span class="o">(</span><span class="n">line</span> <span class="o">-></span> <span class="n">line</span><span class="o">.</span><span class="na">split</span><span class="o">(</span><span class="s">" "</span><span class="o">))</span> |
| <span class="c1">// emit a pair <word,1> for each array element</span> |
| <span class="o">.</span><span class="na">flatMap</span><span class="o">((</span><span class="n">String</span><span class="o">[]</span> <span class="n">wordArray</span><span class="o">,</span> <span class="n">Collector</span><span class="o"><</span><span class="n">Tuple2</span><span class="o"><</span><span class="n">String</span><span class="o">,</span> <span class="n">Integer</span><span class="o">>></span> <span class="n">out</span><span class="o">)</span> |
| <span class="o">-></span> <span class="n">Arrays</span><span class="o">.</span><span class="na">stream</span><span class="o">(</span><span class="n">wordArray</span><span class="o">).</span><span class="na">forEach</span><span class="o">(</span><span class="n">t</span> <span class="o">-></span> <span class="n">out</span><span class="o">.</span><span class="na">collect</span><span class="o">(</span><span class="k">new</span> <span class="n">Tuple2</span><span class="o"><>(</span><span class="n">t</span><span class="o">,</span> <span class="mi">1</span><span class="o">)))</span> |
| <span class="o">)</span> |
| <span class="c1">// group and sum up</span> |
| <span class="o">.</span><span class="na">groupBy</span><span class="o">(</span><span class="mi">0</span><span class="o">).</span><span class="na">sum</span><span class="o">(</span><span class="mi">1</span><span class="o">)</span> |
| <span class="c1">// print</span> |
| <span class="o">.</span><span class="na">print</span><span class="o">();</span></code></pre></div> |
| |
| <h3 id="compiler-limitations">Compiler Limitations</h3> |
| <p>Currently, Flink only supports jobs containing Lambda Expressions completely if they are <strong>compiled with the Eclipse JDT compiler contained in Eclipse Luna 4.4.2 (and above)</strong>.</p> |
| |
| <p>Only the Eclipse JDT compiler preserves the generic type information necessary to use the entire Lambda Expressions feature type-safely. |
| Other compilers such as the OpenJDK’s and Oracle JDK’s <code>javac</code> throw away all generic parameters related to Lambda Expressions. This means that types such as <code>Tuple2<String,Integer</code> or <code>Collector<String></code> declared as a Lambda function input or output parameter will be pruned to <code>Tuple2</code> or <code>Collector</code> in the compiled <code>.class</code> files, which is too little information for the Flink Compiler.</p> |
| |
| <p>How to compile a Flink job that contains Lambda Expressions with the JDT compiler will be covered in the next section.</p> |
| |
| <p>However, it is possible to implement functions such as <code>map()</code> or <code>filter()</code> with Lambda Expressions in Java 8 compilers other than the Eclipse JDT compiler as long as the function has no <code>Collector</code>s or <code>Iterable</code>s <em>and</em> only if the function handles unparameterized types such as <code>Integer</code>, <code>Long</code>, <code>String</code>, <code>MyOwnClass</code> (types without Generics!).</p> |
| |
| <h4 id="compile-flink-jobs-with-the-eclipse-jdt-compiler-and-maven">Compile Flink jobs with the Eclipse JDT compiler and Maven</h4> |
| |
| <p>If you are using the Eclipse IDE, you can run and debug your Flink code within the IDE without any problems after some configuration steps. The Eclipse IDE by default compiles its Java sources with the Eclipse JDT compiler. The next section describes how to configure the Eclipse IDE.</p> |
| |
| <p>If you are using a different IDE such as IntelliJ IDEA or you want to package your Jar-File with Maven to run your job on a cluster, you need to modify your project’s <code>pom.xml</code> file and build your program with Maven. The <a href="http://flink.apache.org/docs/master/quickstart/setup_quickstart.html">quickstart</a> contains preconfigured Maven projects which can be used for new projects or as a reference. Uncomment the mentioned lines in your generated quickstart <code>pom.xml</code> file if you want to use Java 8 with Lambda Expressions.</p> |
| |
| <p>Alternatively, you can manually insert the following lines to your Maven <code>pom.xml</code> file. Maven will then use the Eclipse JDT compiler for compilation.</p> |
| |
| <div class="highlight"><pre><code class="language-xml"><span class="c"><!-- put these lines under "project/build/pluginManagement/plugins" of your pom.xml --></span> |
| |
| <span class="nt"><plugin></span> |
| <span class="c"><!-- Use compiler plugin with tycho as the adapter to the JDT compiler. --></span> |
| <span class="nt"><artifactId></span>maven-compiler-plugin<span class="nt"></artifactId></span> |
| <span class="nt"><configuration></span> |
| <span class="nt"><source></span>1.8<span class="nt"></source></span> |
| <span class="nt"><target></span>1.8<span class="nt"></target></span> |
| <span class="nt"><compilerId></span>jdt<span class="nt"></compilerId></span> |
| <span class="nt"></configuration></span> |
| <span class="nt"><dependencies></span> |
| <span class="c"><!-- This dependency provides the implementation of compiler "jdt": --></span> |
| <span class="nt"><dependency></span> |
| <span class="nt"><groupId></span>org.eclipse.tycho<span class="nt"></groupId></span> |
| <span class="nt"><artifactId></span>tycho-compiler-jdt<span class="nt"></artifactId></span> |
| <span class="nt"><version></span>0.21.0<span class="nt"></version></span> |
| <span class="nt"></dependency></span> |
| <span class="nt"></dependencies></span> |
| <span class="nt"></plugin></span></code></pre></div> |
| |
| <p>If you are using Eclipse for development, the m2e plugin might complain about the inserted lines above and marks your <code>pom.xml</code> as invalid. If so, insert the following lines to your <code>pom.xml</code>.</p> |
| |
| <div class="highlight"><pre><code class="language-xml"><span class="c"><!-- put these lines under "project/build/pluginManagement/plugins/plugin[groupId="org.eclipse.m2e", artifactId="lifecycle-mapping"]/configuration/lifecycleMappingMetadata/pluginExecutions" of your pom.xml --></span> |
| |
| <span class="nt"><pluginExecution></span> |
| <span class="nt"><pluginExecutionFilter></span> |
| <span class="nt"><groupId></span>org.apache.maven.plugins<span class="nt"></groupId></span> |
| <span class="nt"><artifactId></span>maven-compiler-plugin<span class="nt"></artifactId></span> |
| <span class="nt"><versionRange></span>[3.1,)<span class="nt"></versionRange></span> |
| <span class="nt"><goals></span> |
| <span class="nt"><goal></span>testCompile<span class="nt"></goal></span> |
| <span class="nt"><goal></span>compile<span class="nt"></goal></span> |
| <span class="nt"></goals></span> |
| <span class="nt"></pluginExecutionFilter></span> |
| <span class="nt"><action></span> |
| <span class="nt"><ignore></ignore></span> |
| <span class="nt"></action></span> |
| <span class="nt"></pluginExecution></span></code></pre></div> |
| |
| <h4 id="run-and-debug-flink-jobs-within-the-eclipse-ide">Run and debug Flink jobs within the Eclipse IDE</h4> |
| |
| <p>First of all, make sure you are running a current version of Eclipse IDE (4.4.2 or later). Also make sure that you have a Java 8 Runtime Environment (JRE) installed in Eclipse IDE (<code>Window</code> -> <code>Preferences</code> -> <code>Java</code> -> <code>Installed JREs</code>).</p> |
| |
| <p>Create/Import your Eclipse project.</p> |
| |
| <p>If you are using Maven, you also need to change the Java version in your <code>pom.xml</code> for the <code>maven-compiler-plugin</code>. Otherwise right click the <code>JRE System Library</code> section of your project and open the <code>Properties</code> window in order to switch to a Java 8 JRE (or above) that supports Lambda Expressions.</p> |
| |
| <p>The Eclipse JDT compiler needs a special compiler flag in order to store type information in <code>.class</code> files. Open the JDT configuration file at <code>{project directoy}/.settings/org.eclipse.jdt.core.prefs</code> with your favorite text editor and add the following line:</p> |
| |
| <div class="highlight"><pre><code>org.eclipse.jdt.core.compiler.codegen.lambda.genericSignature=generate |
| </code></pre></div> |
| |
| <p>If not already done, also modify the Java versions of the following properties to <code>1.8</code> (or above):</p> |
| |
| <div class="highlight"><pre><code>org.eclipse.jdt.core.compiler.codegen.targetPlatform=1.8 |
| org.eclipse.jdt.core.compiler.compliance=1.8 |
| org.eclipse.jdt.core.compiler.source=1.8 |
| </code></pre></div> |
| |
| <p>After you have saved the file, perform a complete project refresh in Eclipse IDE.</p> |
| |
| <p>If you are using Maven, right click your Eclipse project and select <code>Maven</code> -> <code>Update Project...</code>.</p> |
| |
| <p>You have configured everything correctly, if the following Flink program runs without exceptions:</p> |
| |
| <div class="highlight"><pre><code class="language-java"><span class="kd">final</span> <span class="n">ExecutionEnvironment</span> <span class="n">env</span> <span class="o">=</span> <span class="n">ExecutionEnvironment</span><span class="o">.</span><span class="na">getExecutionEnvironment</span><span class="o">();</span> |
| <span class="n">env</span><span class="o">.</span><span class="na">fromElements</span><span class="o">(</span><span class="mi">1</span><span class="o">,</span> <span class="mi">2</span><span class="o">,</span> <span class="mi">3</span><span class="o">).</span><span class="na">map</span><span class="o">((</span><span class="n">in</span><span class="o">)</span> <span class="o">-></span> <span class="k">new</span> <span class="n">Tuple1</span><span class="o"><</span><span class="n">String</span><span class="o">>(</span><span class="s">" "</span> <span class="o">+</span> <span class="n">in</span><span class="o">)).</span><span class="na">print</span><span class="o">();</span> |
| <span class="n">env</span><span class="o">.</span><span class="na">execute</span><span class="o">();</span></code></pre></div> |
| |
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